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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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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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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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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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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 <108 rpm per rpm (0.03 sec –2). Refer block diag. for details. Actual speed (nact) Measurement: Actual speed nact is acquired by three digital speed pickups (Hall probes) in the form of pulses /frequency. Channel-2 is utilized while other two-channel pick-up remains redundant; electronically switching ensures no affect in channel in service and also a full - proof monitoring. The selected sensed speed channel signal is further divided into three measuring signals (f/v of 0-60 Hz, low range 0-6 Hz & full range 0-60 Hz and a quartz frequency standard) for various other applications in the EHG and other circuits. KORBA SIMULATOR
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The difference of actual speed and time dependent speed signals (nact - ‘nRTD) form the input error of the Speed controller which outputs control signal (in the path as explained in selection section) through the selection modules for driving the EHC and finally establishing the EHG.
ACTUAL SPEED MEASUREMENT / FORMATION
The speed controller is poportional+derivative (P-D) action controller, with sloping characteristics. During steady-state condition, the speed controller outputs for better load sharing by more nos. of turbo-generators connected in the grid as compared with purely mechanical and Hydraulic Governor run T-G sets. Due to proportional
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control, a control error (off-set) is obvious in the speed reference but it does not matter much. An identical speed at synchronising point is possible to be achieved due to a Pre-feed function of pressure. No load correction of speed is achieved by a feed forward signal that is obtained from Boiler pressure controller during synchronising the T.G. set.
Load Control Mode Load reference value pR is generated by means of a reference value setter module as described in speed control mode and is derived manually by the operator adjusted (lower/raise) values by means of a remote driven motorized potentiometer, Load controller is switched ON for bringing load controller in service., it can also be varied by the Automatic dispatch control (When switched ON ) ; it is also termed as pR ADS ,and is basically the MW demand generated by the Coordinated Master Control loop(CMC). The load demand signal is restricted within upper and lower MW loading limits as detailed in CMC loop description. ADC influence ‘ON’ appears when there is NO ADC fault if is selected. In case if, CMC or ADC gets faulted, it is automatically switched OFF; at this situation, matching or the follow up is automatically taking place and loading of the TG set is subsequently made in standby basis.
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Load reference pR as obtained from the potentiometer in voltage signal, is fed to the high gain amplifier whose gain is adjusted by the rate of loading dp/dt or the load margins (In order to load the turbo-generator at highest possible rate consistent to permissible level of thermal stress, the upper/lower load margins are computed by TSE, explained separately.). The Load gradient (load rate dp/dt.) is selected either by On/Off push button or by follow-up command, for inclusion and it either modifies the high gain amplifier slope in both negative and positive sides of the amplifier or the TSE computed margins as explained above modify the high gain amplifier through minimum selector. Upper release margin can result reduction of generated power and lower release margin can result unloading. Time dependent reference signal also referred as ‘pRLIM’’: The Time dependent reference signal also referred as ‘pRLIM’’ is generated through a high gain amplifier and an integrator functioning in fast, normal and stop modes. It follows the ramp characteristic. The proportional leg of the response of the pRLIM can be adjusted between 0-20% of MCR power of the TG Set. The response of the pRLIM is purely integral, if the rate of rise of the pRLIM is limited to the load gradient selected, and at this situation the proportional channel is switched off. This pRLIM rises during start-up at a rate (Mw/min ) selected through load gradient setter until final value. The pRLIM module is continuously allowing matching of the actual power output as long as the generator breaker is open (not synchronized) and ensures smooth transition of speed (during start-up) to load controller (after synchronization). The characteristic of pRLIM is linear 0-10v rising in 04 minutes. And it (pRLIM) acts directly on the load controller without any intermediate control device. At conditions when TG is not synchronised, power error (Pr-Pact) signal governs the follow-up /tracking as explained below. Tracking or the follow-up conditions in the load controller: When the generator C.B. is on but the speed controller has taken over (due to conditions of follow-up) and speed controller remains in action until load controller signal (pr) < (nr) of speed controller; then tracking gets released as soon as (pr) = (nr) and When load shedding is less than station auxiliary power (p act < station load) and Mw error reaches to more than 5 % or the generator circuit breaker is not made on; the load controller output, tracks to speed signal. Stop signal in load control: A ‘Blocking’ or STOP command gets initiated at conditions shown below then the integrator stops further integrating and pRLIM (the load demand) remains steady until the blocking signals are cleared or restored. The block conditions are met at conditions as given below: •
TSE switch is ON (selected) and it goes out of order (got faulted); it is enabled again after the TSE is reset and it becomes O.k.. KORBA SIMULATOR
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•
The load reference signal has been raised and the pressure controller goes/switches to limit pressure mode of operation.
•
The STOP set point binary condition is/gets introduced.
A highly and sensitive linear response with respect to power grid frequency is effected by having the additional load reference component pR∆f (this can be set as low as possibly up to 5 milli Hz); it can be included for operating the load controller with frequency influence included in the system. This frequency influence was being excluded in the system sometimes in nineties, because the units were been operated at very high differentials of frequencies when frequency used to rise to approx 52 Hz at off-peak hours (in night hours) or reduce/decline as low as 47.5 Hz at peak load hours. But now due to insistence by Load Dispatch Center (LDC) to regulate the grid frequency at very tight margin and in order to run systems on ABT mode, the frequency influence inclusion have become mandatory. This is being referred as FGMO operation of units. The load reference thus derived is fed to a minimum selector, which also is fed with the load limiter output. In order to restrict loading, Load limiter is preselected; the Load limit value (due to plant conditions) can be adjusted by the load limiter potentiometers and can also be seen on control desk. Even the reduction in grid frequency cannot cause the TG set to exceed the preset power level due to load limiting. The output signal from the load limiter at the minimum selector in the form of the pR , that is the sum of all reference values acting on the load controller as reference signal or the desired value. Actual Load signal is acquired threefold by means of the load measuring device and transmitted to the controller comparator module but in case the signals of the three parallel cannels deviate by more than 5% an alarm ‘ACTUAL LOAD SIGNAL FAULTY’ along with group alarm of ‘Turbine Controller Faulty’ appears. The difference of the actual measured power signal (pact) and the pR form the input of the load controller that outputs control signal and passes through a selection module for driving the EHC as explained in the admission control and selection diagram. Load Controller consists of two plug-in modules first one to accommodate isolated grid detection and the second to accommodate dynamic loading of the generators & to housing the tracking module. Load Controller is a proportional (P) + integral (I) controller to take care of small changes of load in Proportional mode and large changes in Integral mode operation. Due to this addition, the response of the controller is proportional for small changes of the load reference value but for the large changes of reference value proportional plus integral mode refines the system operation. In order to effect smooth transition from speed controller to Load Controller (Generator breaker open condition i.e. turbine not synchronised with grid) pR is
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compared continuously with pact and control signal is matched ensuring bump less switching. During the time the speed controller is in control for start-up, shut-down or no load operation of TG set, the starting time constant of the TG set remains in dominating whereas during synchronised operation, the transfer functions of generator and the power grid become of vital importance for controller optimisation. Pressure Control Mode The pressure controller controls the turbine load with respect to the main steam throttle pressure and prevents the large pressure drop during fast loading (Quick load increase). The actual steam throttle pressure is measured in turbine area and pressure reference is derived from CMC loop, after comparison the deviated control signal (hRprc) is fed to the Proportional +Integral (PI) action type Pressure controller and its final output is fed to the minimum selector-2 as described earlier in speed controller and load controller loops. The Pressure Controller functions in two modes of operation: • Initial pressure mode • Limit pressure mode Initial Pressure Mode: In Initial pressure mode of operation, constant initial pressure (turbine inlet throttle pressure) is maintained and acts in proportional to pressure setting by minimizing the pressure error (Actual-Ref) even up to zero value. The power delivered by the TG set is determined by the boiler capability up to a maximum of power level as set by load controller; increase of load above this is blocked thus, because it is connected to a minimum selector. The difference pressure ∆p between the reference and actual value, is controlled up to a value of 10 Kg/cm2 which is equal to the pressure drop of steam flow from boiler to the turbine control valve; it therefore ensures natural differential pressure of the steam flowing from boiler to the turbine. A preset potentiometer equivalent to this pressure generates negative voltage to the controller input and it biases the pressure differential ∆p thus in the controller. Limit pressure mode: Limit pressure mode uses the boiler storage capacity and is effected either by push button or gets automatically selected as soon as the pressure deviates to 10 Kg/cm2 from normal running pressure to operate the controller in Limit pressure mode. This deviation of 10 Kg/cm2 pressure signal already subtracted in the in the input of Pressure controller, as described in the initial pressure mode, gets neutralized by automatic switching. Introducing the Limit Pressure Operation is therefore possible to regulate boiler pressure beyond a pre-set pressure of main steam and load in small or quick variations, and is controlled until pre-set pressure is reached that is not possible in normal frequency based load control.
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In fact the normal ∆p from boiler to turbine (as explained in initial pressure mode) is not persisting either due to increase in pressure at turbine side due to load throw, vacuum drop, extractions’ closure etc or due to pressure drop in boiler side by nonavailability/reduced loading of mills, of coal feeders or any other causes. At this situation due to equating/reducing of differential pressure ∆p , an alarm is generated so as to warn/alarm the operator of the discrepancy. When Limit pressure engaged alarm appears, the stop signal in the load control loop is also generated for blocking the pRLIM signal from increasing/reducing.
All the three controllers are operative in such a manner that the governing of T.G is ensured full proof and speeding or loading of the T.G. is best maintained as per the pressure in the system and Boiler or turbine follow mode is achieved with full reliability and safety. Co-ordinated Master Control (CMC) ensures co-ordination between Boiler & Turbine. The Co-ordinated Master Control (500 Mw) block diagram has been given below, we find that the Unit master receives load demand signal from load dispatcher (ALC). A GNI computer/SPCM is provided with the system to decide target value Z0, Run Back load limits & load rate required for proper generation, Boiler master controller, Turbine load set-point etc., through which the CMC is ensured. The load demand signal as generated in CMC, for turbine control reaches to point ‘D’ of EHG block diagram (refer the load control mode) and is switched for inclusion to operate the
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EHG in coordination with grid dispatch ADC demand. Boiler Follow or Turbine follow modes are decided by switching suitably and loaded TG operation is achieved as explained in details of CMC mode of integrated control in C&I , ACS section.
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TURBINE PROTECTION SYSTEM Turbine protection system performs to cover the following functions: a. Protection of turbine from inadmissible operating conditions. b. In case of plant failures, protection against subsequent damages. c. It restricts occurring failures to minimum. Standard turbine protection system comprises the following: Mechanical/hydraulic turbine protection. Electrical turbine protections.
BLOCK DIAGRAM OF TURBINE PROTECTION AND ATT
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Mechanical Hydraulic Turbine Protection The design of mechanical hydraulic protection equipment is in accordance with hydraulic break current principle and consists of following: a. Two manual trip devices (main trip valves) b. Two speed monitors (over speed trip device) c. One hydraulic low vacuum device d. Two solenoid valves for trip initiation (remote solenoid valves) As explained earlier, turbine stop and control valves are tripped to close position if the trip oil pressure is reduced below the minimum value. The main trip valves allow rapid draining of trip oil in case they are operated either manually or automatically by the reduction of aux. trip oil pressure. Aux. trip oil pressure can be drained because of actuation of hydraulic low vacuum trip device, over speed trip device or thrust bearing trip device. The principle of functioning of individual hydraulic trip devices is explained in details under the chapter of Automatic Turbine Testing System. Remote trip solenoids act as interfaces between mechanical hydraulic and electrohydraulic protection equipment of turbine. Upon receiving the electrical trip command, the solenoids get energised and close the valves. Thus control oil supply to main trip values is cut off leading to their closure. Electrical Hydraulic Turbine Protection Electrical turbine trip equipments comprise two-channel redundancy and function on operating current principle. All electrical trip criteria act on the two remote trip solenoid valves to energise the solenoids. The electro-hydraulic turbine protection equipment features - Two solenoid operated valves for trip initiation (Remote trip solenoids). - Emergency trip contactor cabinet containing trip channels 1 and 2 - Monitors with signal conditioning - One substitute channel to ensure uninterrupted transmission of eventual turbine trip signals during testing by ATT. The remote trip solenoids (RTS) have already been described. Operation of any one channel causes energising both solenoid-operated valves leading to turbine trip eventually. Transmitters that cause a trip in the case of any electrical tripping signal are conditioned and monitored via binary signal conditioning of the ATT system or via the central analog/binary signal conditioning.
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TURBINE PROTECTION FOR 200MW KWU SETS
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Turbine Trip Actuation Circuits The turbine protection system is sub divided into two parts: a. Protective circuits for the standard turbine protection equipments or criteria. b. Protective criteria from other areas. Standard criteria are specified by the turbine manufacturer and are responsible for full protection of turbine under various specific conditions, which are: 1.
Manual tripping devices (Turbine trip gear local operating lever)
2.
Speed monitors (over speed trip devices)
3.
Thrust bearing trip device
4.
Hydraulic low vacuum trip device
5.
Electrical low vacuum trip device
6.
Lub oil pressure protection
7.
Fire protection
8.
Manual turbine tripping (electrical UCB switch)
Protection criteria from other areas are as follows: •
Boiler trip (MFR)
•
Boiler drum level very high ( > + 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
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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
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x
x
Standstill
x
x
x
Standstill
x
x
Standstill
x
x
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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
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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 <8
3 2 2
60 %
Max temperature of cooling water outlet
36 oC. 43 oC.
Hot gas temperature
75 oC (Alarm)
Nominal gauge pressure at winding inlet
3.09 Kg/cm
Max temperature of Stator Water at winding inlet
36 oC
Max temperature of Stator Water at winding outlet
70 oC
Max temperature of cooling water inlet
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Stator water flow 3
Normal
27 ± 3 m /hr
Alarm
21 m /hr
Trip
13 m /hr
3 3
Stator water conductivity Normal
< 5.0 micro mho/cm
High
13.3 micro mho/cm
Trip
20.0 micro mho/cm
Stator water expansion tank Vacuum
200-300 mm of WCL
Auto start of standby SW pump
2.4 Kg/cm
2
Nominal consumption of cooling water At 35 oC. At 37 oC.
95 m /hr
At 40 oC.
130 m /hr
3
3
110 m /hr 3
Safety Valve release (A.C. seal oil pump)
9 Kg/cm
2
Safety valve release (D.C. seal oil pump)
9 Kg/cm
2
Seal oil temperature after Seal oil cooler Normal
20 - 40 0C.
Alarm
45 0C.
Seal oil outlet temperature Normal
40 0C.
Alarm
65 0C.
Differential pressure across duplex filter Normal
0.4 Kg/cm
2
Alarm
0.6 Kg/cm
2 2
Seal oil pressure at Turbine and Slip ring end
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Permissible temperature rating of Generator: High
V. High
1.
Generator bearing and seal Babbitt temperature
75 0C.
85 0C.
2.
Generator bearing oil outlet temperature
60 0C.
70 0C.
3.
Stator winding temperature
75 0C.
105 0C.
4.
Rotor winding temperature
110 0C.
115 0C.
5.
Stator core temperature
95 0C.
6.
Hot gas temperature
75 0C.
7.
Cold gas temperature
44 0C.
OPERATION LIMITS Capability of the Generator The generator is capable of delivering 247 MVA continuously at 15.75 KV terminal 2 voltage and stator current 9050 A, at a Hydrogen pressure (g) of 3.5 Kg/cm . The cold gas temperature not to exceed 44 0C and distillate temperature at inlet of stator winding not to exceed 45 0C. Output of the generator at various lagging and leading power factors are as per the generator capability curve. Variation of Terminal Voltage
Generator can develop rated power at rated power factor when the terminal voltage changes within ± 5% of the rated value i.e. 14.96 KV to 16.54 KV. The stator current should accordingly be changed within limits of 5% ± i.e. 8600 A to 9500 A. Permissible voltage of operation and corresponding values of the MVA outputs of stator currents are given in Table - 1. During continuous operation of the generator at 110% of the rated value stator current should not increase beyond 9500 A corresponding to 105% of the rated value.
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TABLE – I Terminal 17.32 voltage in KV Output in 217 MVA Stator current 7.24 in KA
17.17
17.01
16.85
16.7
16.54
15.75
14.96
224.7
231
237
242
247
247
247
7.56
7.92
8.14
8.37
8.6
9.05
9.5
During continuous operation of the generator at 110% of the rated value stator current should not increase beyond 9500 A corresponding to 105% of the rated value.
Frequency Deviation The Generator can be operated continuously at rated output with a frequency variation of ±5% of the rated value i.e. 47.5 Hz. to 53.5 Hz. However, the performance of the generator with frequency variation is limited by the turbine capability.
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Temperature of the Coolants If the temperature of the cooled hydrogen or inlet water to gas coolers increases beyond the rated value, the unloading of the generator has to be carried out as per the given curves. The operation of the generator with cold gas temperature more than 55 0C is not permitted. Operation of the generator with cold gas temperature below 20 0C is not recommended. Similarly, if cold distillate temperature at inlet of stator windings increases beyond the rated value, unloading of the generator has to be carried out as per the curves. The operation of the generator with cold distillate temperature more than 48 0 C is not permitted. Operation of the generator with cold distillate temperature below 35 0 C is not recommended.
UNLOADING SCHEDULE DUE TO HIGH DISTILLATE TEMPERATURE
Overloading Under normal conditions, the generator can be over loaded for short duration. Permissible values of short time over loads of Stator Current Vs Time in minutes and Rotor Current Vs Time in seconds are given in Table - II and Table - III respectively.
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TABLE – II Stator current in KA
13.57
12.67
12.22
11.76
11.31
10.86
10.41
9.95
Time in minutes
1
2
3
4
5
6
15
60
TABLE - III Rotor current in KA
5.2
3.9
3.12
2.73
Time in seconds
20
60
240
360
Operation under Unbalanced Load The turbo-generator is capable of operating continuously on an unbalanced system loading provided that continuous negative phase sequence current during this period shall not exceed 5% of the rated stator current i.e. 452.5 A. It implies that maximum difference between limit current is about 10% of the rated value. At the same time current of maximum loaded phase should not exceed the permissible value for normal conditions of operation of turbo-generator under balanced loading. If the unbalance exceeds the above permissible limits, measures shall be taken immediately to eliminate or reduce the extent of unbalance within 3 to 5 minutes. In case it is not possible to achieve this, the machine has to be run down and tripped. If negative sequence current reaches a value of 20-25% of the rated value trip-relay will operate and the generator will be automatically tripped. Asynchronous Operation Asynchronous Operation of the generator on field failure is allowed depending upon the permissible degree of the voltage dip and acceptability of the system from the stability point of view. During field failure, the field suppression shall be cut out from the circuit and active load on the generator shall be decreased to 60% of the rated value within 30 seconds and to 40% in the following 1.5 minutes. The generator can operate at 40% rated load asynchronously for a total period of 15 minutes from the instant of excitation failure. Within this steps should be taken to establish the reasons of field failure to restore normalcy or the set should be switched over to reserve excitation. Unsymmetrical Short Circuit Performance The duration of unbalanced operation should be such that the product of square of 2 negative sequence component of current I 2 expressed in terms of per unit value of 2
stator current and its duration in seconds does not exceeds 8 (I 2 t < 8).
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NEGATIVE SEQUENCE CURRENT CAPABILITY CURVE
The permissible value of negative sequence current and the corresponding durations are given in Table - IV. TABLE – IV Duration of short circuit in seconds
1.2
5
10.9
Negative sequence Current
2.5
1.25
0.9
Operation at Reduced Hydrogen Pressure Continuous operation of the turbo-generator with lower hydrogen pressure than the 2 rated value of 3.5 Kg/cm is not permitted. However, during emergency, the generator can be run at reduced hydrogen pressure with reduced load for a short duration as given in Table - V. TABLE - V H2 Pres. kg/cm2 (g)
Generator Load (MW)
Duration of Operation
3.0
200
Continuous
2.0
115
Continuous
1.0
30
Continuous
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Within this time action should be taken to restore the hydrogen pressure to normal value. Operation of generator with air-cooling is NOT PERMITTED. Capacity of the Generator with One Cooler out of Service The generator can deliver 185 MW continuously when one gas cooler is out of service. The operation of the generator with more than one cooler out of service is not permitted. Refer to Fig
Motoring Action Motoring of the turbo-generator is permissible within the limitation of the turbine.
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STATIC EXCITATION SYSTEM Description Static Excitation System is used in most of the 200 MW Generator sets. The AC power is tapped off from the generator terminal, stepped down and rectified by fully controlled thyristor bridges and then fed to generator field as excitation power, to control the generator-output voltage. A high control speed is achieved by using an inertia free control and power electronic system. Any deviation in generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of thyristor thereby controlling the field excitation. The static excitation system consists of: i.
Rectifier Transformer
ii.
Thyristor Converter
iii. Automatic Voltage Regulator iv.
Field flashing Circuit
v.
Field breaker and field discharge equipment.
Rectifier Transformer The power transformer gets input supply from the generator output terminals. The secondary is connected to the Thyristor Bridge, which delivers a variable DC output to the generator field. Normally it is a dry type transformer. Thyristor Converters The converter is assembled in one or more numbers of cubicles depending on the number of thyristor bridges connected in parallel. The number of bridges is so designed that in case one bridge fails during operation, the remaining bridges will have adequate capacity to feed the generator field for full load output. Fans mounted on the top of the cubicles cool the thyristor bridges. Field Flashing Circuit Since it is difficult to start the excitation system with the residual voltage at nominal speed, a field flashing circuit is provided to overcome this problem. Initially the station auxiliary supply of 415 volts is stepped down by a small transformer and then rectified in a rectifier bridge and supplied to the generator field. As soon as the generator output builds up to 40%, the excitation system starts working smoothly. A back-up battery supply is given in parallel to field flashing output.
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FIELD BREAKER CUBICLE For rapid de-excitation of synchronous machine and complete isolation of the field from the Thyristor Bridge, a field breaker is provided. In case of electrical faults, the field breaker provides protection by isolating DC source from the field. The magnetic field energy is dissipated through a field discharge resistance.
AUTOMATIC VOLTAGE REGULATOR It is the heart of excitation system. It consists of the following components: ERROR DETECTOR AND AMPLIFIER The generator terminal voltage is stepped down by three phases PTs and fed to the Automatic Voltage Regulator (AVR). The AC input thus obtained is rectified and compared against a highly stabilized reference value and any difference is amplified in different stages of amplification. For parallel running of generators compounding feature is provided. Three CTs sensing the current in the generator terminal feed proportional current across the variable resistors in the AVR. The voltage thus obtained across the resistor can be added vectorially either for compounding purpose or for transformer droop compensation.
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GRID CONTROL UNIT The output of the AVR is fed to a grid control unit. It gets its synchronous AC reference through a filter circuit and generates a row of pulses whose position depends on the DC input from the AVR i.e. the pulse position varies continuously as a function of the control voltage. The pulse limit for rectifier and inverter (deexcitation) operation can be adjusted independent of each other by potentiometer provided on the front side of unit. Six double pulses displaced by 60o from one another are generated at this output. Two relays are provided, by exciting which, these pulses can be either blocked completely or shifted to inverter mode of operation.
PULSE AMPLIFIER The pulse output of this grid control unit is amplified further at an intermediate stage of amplification. This is also known as pulse coupling stage. It has also DC power supply unit which operates from a three-phase 380V supply and delivers + 15V, -15V, +5V and a coarse stabilised voltage UL. A built-in relay is provided which can be used for blocking a 6-pulse channel. In a two-channel system, energising and de-energising the relay affect the changeover. PULSE FINAL STAGE The unit receives input pulse from the previous stage i.e. pulse amplifier (intermediate stage) and transmits them through pulse transformer to the gates of the thyristor. The step pulse at the output ensures simultaneous firing of several thyristor in parallel. A built- in power supply provides the required DC supply (+ 15V +5V & UL) to the final amplifiers.
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Each thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge fails with its final pulse stage, the remaining thyristor bridges can continue to provide full load output and thereby ensure (n-2) operation. Pulses can be blocked with an internal relay provided in this unit. Pulses are blocked in case of Failure of one or more thyristor fuses. -
Failure of power supply of the final stage.
-
Failure of the converter fans.
MANUAL CONTROL CHANNEL A separate manual control channel is provided where the controlling DC signal is taken from a stabilized DC voltage through a motor operated potentiometer. The DC signal is fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to final pulse stage. When one channel is working generating the required pulses, the other remains blocked. Therefore, blocking or releasing the pulses of corresponding intermediate stage affects a changeover between auto and manual control. A pulse supervision unit detects spurious pulses or loss of pulses on the pulse bus bar and transfers control from ' Auto'to ' Manual'channel. FOLLOW-UP UNIT To ensure a smooth changeover from ' Auto'to ' Manual'control it is necessary that the position of the pulses in both the channels should be identical. A pulse comparison unit detects any difference in the position of the pulses and with the help of a follow-up unit in actuates motor operated potentiometer on the manual channel to turn in direction so as to eliminate the difference. However, while transferring control from manual to auto, any difference in the two control levels can be visually checked on the balance meter provided on the control panel and after matching the two signals the changeover can be done. LIMIT CONTROLLERS When a generator is running in parallel with the power network it is essential to maintain it in synchronism without exceeding the maximum permissible load on the machine and also without the protection system tripping. So it is necessary to influence the voltage regulator by suitable means to limit the over-excitation and under-excitation. The following limiters are normally used in the static excitation system. a.
Stator Current Limiter
b.
Rotor Current Limiter
c.
Rotor Angle Limiter
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Stator/Rotor current Limiters limit the control voltage to a value corresponding to the permissible excitation in over excited operation while rotor angle and stator current limiter limits the control voltage to value corresponding to permissible excitation in under excited operation. STATOR CURRENT LIMITER This unit functions in conjunction with an integrator unit, which provides the necessary dead time and the gradient that can be adjusted by potentiometers. The regulator consists of a measuring converter, two comparator, two PID regulators and a DC power pack. A discriminator in the circuit differentiates between inductive and capacitive current. The positive and negative signals processed by two separate amplifiers are brought to the output stage and only that output which had to take care of the limitations is made effective. The inductive current limit is affected through the integrator while the capacitive current gets directly on the AVR output. ROTOR CURRENT LIMITER The unit basically comprises an actual value converter, a limiter with adjustable PID characteristics, a reference value, dv/dt sensor and a signaling unit. The field current is measured as the AC input side of the thyristor converter and is converted into proportional DC voltage. The signal is compared with an adjustable reference, amplified, and with necessary time lapse, fed to the voltage regulator input. The limit is reduced during (n-2) operation through a relay switched circuit. Also, during a fault condition (when dv/dt is large and -ve), the limit is raised (field forcing limit). ROTOR ANGLE LIMITER This unit limits the load angle i.e. the angle between the voltage of the network centre and the rotor voltage. The limiter is fed with generator terminal voltage & current and through a simulation circuit derives the rotor voltage & grid voltage & hence the load angle. It comprises an actual value converter, a limiting amplifier with adjustable PID characteristics and a reference value unit. The limiting regulator operates as soon as the DC value exceeds the reference value. SLIP STABILISING UNIT The slip stabilising unit is used for the suppression of rotor oscillations of the alternator through the additional influence of excitation. The slip as well as acceleration signals needed for the stabilisation are derived from active power delivered by the alternator. Both the signals are amplified and summed up to influence the excitation of the synchronous machine through AVR in a manner so as to suppress the rotor oscillation.
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EXCITATION SYSTEM PROTECTION Excitation Transformer Protection The protection unit for the excitation transformer is normally mounted on the swing frame of the regulator cubicle. It consists of two over current relays, with adjustable ranges. The current supply for the relays is made through a DC converter, which receives its input supply from battery through a filter circuit. Besides over-current protection a dry type rectifier transformer is embedded with temperature dependent resistance at the low voltage windings. With rise in temperature the resistance value changes sharply after a certain level. This change with one resistor is used for 'warning'and with another for ' tripping' .
Converter Protection Fuses Each thyristor in the converter is connected with a fast acting semiconductor fuse to protect in case of over-current.
Resistor/Capacitor Network across to each thyristor for protection against hole storage effect.
Airflow Monitoring Since converters are air-cooled by fans the airflow is monitored by airflow relays.
Redundancy The thyristor bridges are designed such that in case of failure of one, the remaining bridges will be adequate to provide full load output with field flashing.
Isolator Isolators are provided on the input and output side of the converters to enable replacement of defective thyristor under running load.
AVR Protection All D.C. power supplies receive their input AC supply through Miniature Circuit Breaker with thermal overload relays. Failure of protection and control voltage, as also Regulation Supply, result in tripping of the Field Breaker. Failure of Auto Power Supply / Regulation supply result in to Manual channel change over.
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Power chart of a Turbogenerator (Solid rotor design)
SIGNIFICANCE OF MACHINE CAPABILITY DIAGRAM Capability diagram of the generator gives the safe operating domains. It gives the basic information regarding the limiting zones of the operation so that limiter can be set / commissioned suitably for safe operation of unit.
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EXCITATION SYSTEM PARAMETERS ( KORBA STAGE-1 ) GENERATOR MVA
MW
VOLTAGE
STATOR CURRENT
POWER FACTOR
SPEED
247
210
15.75 KV
9050 A
0.85 LAG
3000 RPM
EXCITATION SYSTEM Ifn
:
2512 A
Vfn
: 300 V
Ifmax
:
3000 A (contin.)
Ifo
: 917A
Vfo
:
120 V
Field resistance
:
0.0896 Ω
Field forcing
:
1.4 Ifn
(n-2) limitation
:
0.65 Ifn, 1.1 Ifn
α min
:
30o
α max
:
72o
Over voltage setting
:
2.5 KV
Rotor Earth fault
:
5 K Ohm, 2.2 K Ohm.
Range of control of AVR
:
Field flashing off
:
30% (85% - 115%) Blocking of Ch. pulses upto 30% voltage 70% V
Excitation transformer
:
( 3900 A, 620 V for 10 sec )
15.75 KV/575 V, 2500 KVA,
CTs Generator
:
10,000/5 A
Excitation Transformer
:
200/1 A, 2500/5 A
DCCT
:
3000/2 A
PTs
:
15750/110 V
DCPT
:
600V/20 mA
Field flashing transformer
:
5 KVA, 415/35 V
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Field Breaker closing 1 GE2 Open 2
386GX Reset
Field Flashing
:
Up to 70% voltage within 20 Secs + RPM Relay (2950 rpm)
Field Breaker Tripping Protections 1
Class A 30Z → 86G→ 386 GX → Field Breaker Trip
a.
Transformer Over Current Instantaneous
b.
Transformer Over Current delayed 2nd stage
c.
Rotor Over Voltage
d.
48V supply fail
e.
Three or more (≥/3) failed i.
ii.
Fan fail a.
Air Flow Relay
b.
Fan supply
Pulse final stage power supply failed
iii. Thyristor fuse failed iv. 2
Isolator open.
Class B 30F → UTR → LFPR(32GI) → 32GI→ 2/32G1 → 86G → 386G → FB Trip
3
a.
Rotor Earth Fault, stage-II
b
Regulator supply fuse fail
c
Manual Channel fail
d
Thyristor Fan supply fuse fail
e
Transformer Temp. HI - HI
Direct Trip Trip the F.B. directly, Gen trips through field failure relay. a
Field Flashing disturbed
b
Switch in Test Position
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4
Automatic Changeover to Manual a
Supply A1 fail
b
Supply A fail
c
Excitation transformer O/C delayed Stage-I
d
Excitation transformer temp HI
e
Auto Channel pulse failure
f
AVR PT Fuse Failure
Alarms a
Protective change over to manual
b
AVR Fault
c
FB Tripped due to AVR fail
d
Limiters in Action
e
Loss of Control voltage
f
Rotor E/F
Limiters a Stator Current Limiter b
Rotor current limiter
c
Rotor Angle Limiter
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GENERATOR PROTECTION Description The core of an electrical power system is the generator. During abnormal operating conditions certain components of the generator are subjected to increased stress and therefore, could fail, referred to as faults. It can be either internal fault or external fault depending upon whether they are inside or outside of the machine. The machine with fault must be tripped immediately. The corrective measures against generator' s abnormal operation are taken care by stubborn protective system. Task of the protective system: - Detect abnormal condition or defect. - Limit its scope by switching to isolate the defect. - Alarm the operating staff. - Unload and/or trip the machine immediately. Requirement of Protective Device: - Selectivity: Only that part of the installation actually containing fault should be disconnected. - Safety against faulty tripping: There should be no trip when there is no fault. - Reliability: The device must always act within the required time. - Sensitivity: Lowest signal input value at which the protective device must act. - Tripping time: There should be clearly a distinction between the tripping time of the device, considering the circumstances such as current and total tripping time for the fault. The total fault clearing time now is of the order of 100 (mill sec.) Protective Devices The choice of the protective equipment for a generator requires precise knowledge of the stress to which the generator is subjected to during services in order that preventive measure may be devised for avoiding inadmissible stress. Important stresses include the electrical voltages to which insulation is exposed, the mechanical forces affecting various parts of the machine and effects of the temperature rise.
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Types of Protections The details of the protective circuits of a 200 MW turbo-generator are given in the above fig. The several faults occurring in generator can be either electrical or mechanical in nature. As such generator protection is broadly segregated into two parts i.e. electrical protection-mostly Class-A type and mechanical protection-mostly Class-B type. ELECTRICAL PROTECTION 1.
Differential Protection: a.
Generator Differential
b. UAT Differential c.
Overhead Line Differential
d. G.T. Restricted Earth Fault, Main e. 2.
Overall Differential
Earth fault protection: a.
Stator Earth Fault
b. Stator Earth Fault, stand by c.
Rotor Earth Fault
3.
Stator Interturn Fault
4.
Negative Phase Sequence Current
5.
Generator Backup Impedance
6.
Loss of Excitation
7.
Pole Slipping
8.
Over Voltage
9.
Over Fluxing
10. Low Forward Power 11. Reverse Power 12. Generator Local Breaker Backup (LBB) 13. Generator Transformer Protections a.
Buchholz Protection
b. PRD Protection c.
Winding Temperature High
d. Oil Temperature High e.
Fire Protection
14. UAT Protection 15. Bus Bar Protection
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DIFFERENTIAL PROTECTION a. GENERATOR DIFFERENTIAL A direct short circuit between different phases of the winding causes a severe fault current flow through the windings and results in extensive damages. As a result there is a distinct difference between the current at the neutral and terminal ends of the particular winding. This difference is detected by differential relay. The current entering and leaving the protected object are determined by current transformers and compared by relays by means of a differential circuit as shown in the figure. A fault inside the protected zone is fed from either one side or both sides depending upon the current sources present, thus producing a difference current in the differential circuit. If this differential current exceeds a set percentage of the current flowing in the protected object, the relay picks up. The relays used is designated 87 G and is CAG 34.5 amp type. It is set to operate 10% (0.5 amp) relay current which corresponds to 1000 amp fault current. b. UATS DIFFERENTIAL Since UATs are connected directly to the stator windings, it has been provided with a biased differential protection in a similar circulating current scheme. The relays are designated 87 UAT and 87 UTB are DTH 31 type. c. G.T. OVERHEAD LINE DIFFERENTIAL The 400 KV bushings of the generator transformer are connected to the switchyard double moose conductor overhead line. Any fault occurring on these lines is detected by overhead line differential protection. The relay designated 87 L is of CAG 34 type 1 amp. G.T. RESTRICTED EARTH FAULT The H.V. winding of the generator transformer is star connected and the neutral is solidly earthed. This protection meant for complete protection of H.V. winding of generator transformer. The delta side of the generator transformer is considered as a part of the generator and its earth fault would cause the earth fault current to flow toward the generator neutral and be detected as generator earth fault. The relay is designated as 64 GT and is CAG 14 type one amp. and a high impedance definite current attracted armature type.
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d. G.T. OVERALL DIFFERENTIAL Since Generator Transformer is directly connected to the stator winding, it would be proper to include the transformer windings associated bus ducts including those for UAT HV side and conductors in a similar circulating current protection scheme. The relay is designated 87 GT and is of DTH 32 type 5 amp which is a biased differential type relay. Biased setting of 30% is used to prevent the relay operation in case of a through fault when the current transformer may saturate and produce an erroneous secondary current.
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EARTH FAULT PROTECTIONS a. STATOR EARTH FAULT (Main) The generator neutral is earthed through the primary winding of neutral grounding transformer of the rating 50 KVA, 15.75/0.24 KV ratios. The secondary winding of the transformer is shorted through loading resistance of 0.42 Ω. For an earth fault in the generator the E/F current flows in the primary of the neutral grounding transformer. As a result a voltage across the resistor is developed which activates stator E/F sensing relay. The reason for this kind of protection is due to mechanical damages resulting from the insulation fatigue, creepage of the conductor bases, vibration of the conductors or other fittings of the cooling systems. The earth fault relay designated is VDG 14 type 64 G1. The relay has a inverse definite minimum time characteristics. Generally 5% Generator winding starting from neutral point remains unprotected because a fault in these portions will generate too low a voltage for relay operation.
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Stator Earth Fault (Main)
b. STATOR STANDBY EARTH FAULT The relay is connected across an open delta of the generator PT secondary windings. When there is no E/F, the sum of the phase voltages of the generator and hence the voltage across the relay is zero. The voltage across the point a & b will assume a positive value when one phase voltage of the generator drops because of earth fault on that phase. The relay is 64 G2, VDG 14 type. It has a inverse time voltage characteristics. c. ROTOR EARTH FAULT Ground leakage in the rotor circuit of a generator does not adversely affect operation, if it occurs only at one point. Danger arises if a second fault occurs causing the current to be diverted in part at least, from the intervening turns, which can burn the conductor causing severe damage to rotor. If a large portion of winding is shorted, the field flux pattern may change causing the flux concentration at one pole and wide dispensation at the other. The attractive force, which is proportional to the square of the flux density, will be stronger at one pole than the other which will cause high vibration and may damage the bearings and may sufficiently displace rotor thereby fouling the stator. Rotor E/F protection is provided by monitoring the I.R value of the rotor winding I.R value < 5.5 KΩ : Alarm , I.R value < 2.2 KΩ : Trip
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STATOR INTER-TURN FAULT When leakage occurs between the turns in the same phase of a winding the induced voltage is reduced and there will be a voltage difference between the centre of the terminal voltage triangle and the neutral of the machine. Therefore, in a generator having one winding per phase, a voltage transformer is connected between each phase terminal and the neutral of the winding, the secondary transformer leads being connected in open-delta, when inter-turn leakage occurs at the ends of the open delta, it is detected by a polarised voltage relay. For generators having several parallel windings per phase, the neutral ends are connected together to form, as many neutrals as there are parallel windings per phase. These neutral are then joined through current transformer to current relays, or through voltage transformer to voltage relay. If an inter-turn fault occurs in the machine, the current transformer carries a transient current or alternatively voltage transformer produce a voltage thereby picking up relay and tripping the Generator. The relay is designated 50 GI, is a CAG 14 type 5 amp attracted armature, definite current operated type.
NEGATIVE PHASE SEQUENCE A three phase balanced load produces a reaction field, which is constant and rotates synchronously with the rotor field system. Any unbalanced condition could be resolved into positive, negative and zero sequence components. The positive sequence component is similar to the balanced load. The zero sequence components do not produce armature reaction. The negative sequence component is similar to that of
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positive sequence but the resulting reaction field rotates in the opposite direction. Hence the flux produced by negative phase sequence current cuts the rotor at double the rotational speed thereby inducing double frequency currents. As a result eddy currents produced are very large and cause severe heating of the rotor windings particularly damper windings. For any current conditions in the three phases the amount of unbalance can be determined from the values of negative sequence components I2 of current by the method of symmetrical components. The degree of unbalance is taken to the value of the negative sequence current component expressed, as percentage of rated current. The losses in the rotor are 2 proportional to the square of the degree of unbalance. This generator has I2 t = 8 2
characteristics indicating that within I2 t < 8, the generator is capable of withstanding but beyond it there is time delay. The time delay has to be matched to the machine negative sequence current withstands capability. The relay used is designated 46G and is of solid-state design and CTNM type.
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GENERATOR BACK-UP IMPEDANCE PROTECTION Three-phase zone impedance is provided for the back-up protection of generator against external three phases and phase to phase faults in 400 KV systems. The zone of impedance relay should be extended beyond 400 KV switchyard and it should be connected to trip the generator after a time delay of 1 to 1.5 seconds so that the generator is tripped only when 400 KV protections has not cleared the fault even in the second zone. The relay used is designated 21G. LOSS OF EXCITATION Failure of the field system leads to losing of Synchronism and resulting in running above synchronous speed. It acts as an induction generator, the main flux being produced by wattless stator current drawn from the system. Operation as an induction generator necessitates the flow of slip frequency current in the rotor; damper winding, and slot wedges, excitation under these conditions requires a large reactive component which approaches the value of rated output of the machine. Since rotor would get over heated due to slip frequency current, the machine should not run more than a few seconds without excitation. Also, it could overload the grid, which may not be able to supply the required excitation MVAR. When loss of excitation is accompanied by under voltage it will initiate Class-A trip. Otherwise Class-B trip if the grid is able to sustain the voltage dip. The relay used is designated 40 G YCGF type. POLE SLIPPING The asynchronous operation of the machine while the excitation is still intact unlike loss of excitation causes severe shock to both machine and grid due to violent oscillations in both active power and reactive power. Because of this the machine may fall out of step or usually known as pole slipping trip. The oscillation may disappear in few seconds; in that case it is not desirable to trip the machine. If however the angular displacement of the rotor exceeds the stability limit the rotor will slip a pole pitch. If this disturbance has been sufficiently reduced by the time this has occurred, the machine may regain synchronism, but if it does not, it must be isolated from the system. The swing curves can be detected by an impedance relay. The relay has two measuring elements set at two values near the independence as seen by the relay. As the impedance seen by the relay changes it comes in the operating zone of the two relays one after the other. The sequential operation is observed by auxiliary relays. Since the system faults would suddenly change the system impedance both the relays shall operate within 55 ms. however, during pole slipping, the two elements would operate sequentially and a trip command is given when both have operated. The relay can be set to be in operation for swings up to +90O corresponding to the stability limits of the unit. The relay used is 98G and is of solid-state design of ZTO type. In order to discriminate against swings on the grid, the tripping is through an impedance relay (98 GY) set with a reach up to the 400 KV yard.
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OVER VOLTAGE The generator winding is rated for 15.75 KV at terminal, sustained over voltage would unduly stress the winding insulation and may lead to failure. To protect the machine against over voltage the protection relay senses the voltage at the secondary of the bus duct PTs. The relay is set to operate at 10% rise in the terminal voltage. A time delay of 3 seconds is provided to take care of transient over voltage arising from line charging, switching capacitive faults etc. The relay used is designated 59 G & is VTU 12 type 110V A.C. G.T. OVER FLUXING The iron core of the generator transformer carries the flux to produce required emf. If the flux increases unduly, the magnetic circuits of the generator and G.T become over saturated resulting in high magnetising current. This in turn leads to higher iron losses, which will increase the winding temp. of the transformer. Since core can be damaged because of this overheating, protection has to be provided against it. The flux is dependent on ratio of voltage & frequency. The condition of over fluxing could arise in case the voltage at the machine terminal rises or its frequency drops or both occurring simultaneously. Practically this condition will arise if the machine AVR misbehaves thereby unduly increasing the voltage even when the grid frequency is low. The relay used is 99 GT and is GTT21 type which senses v/f ratio at the secondary of the bus duct P.T. and gives alarm and trip signals at different time delay. The adopted setting for relay is v/f = 1.2 P.U. i.e. 20% higher than rated v/f ratio. Alarm is at 0.5 - 1 sec & trip at 12 sec in v/f relay & generates AVR ' Raise'block. Surge voltage originating from lines because of switching or atmospheric disturbance is dealt with directly by lighting arrestor and surge diverters. LOW FORWARD POWER PROTECTION When a generator, synchronised with the grid, loses its driving force the generator remains in synchronism. The generator should be isolated from the grid after the steam flow ceases and the flow of power to grid reduces to minimum i.e. the point when the generator starts drawing power from the grid and acts as motor. When the load on generator drops to less than 0.5 percent, generator low forward power relay gets energised and with turbine tripped or stop valves closed, trips the generator with a time delay of 2 seconds. This is a protection to trip generator on other than electrical faults. Also this protection is used for a few electrical faults where generator trip can be delayed. However, provision for time lag unit is there to prevent undesired operation from transient power reversal. The power relay used is designated 32 G1 and is a WCD type.
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REVERSE POWER PROTECTION The generator must be disconnected from the grid as soon as turbine stop/control valves have closed, completely shutting off the steam. Continued full speed turbine rotation causes lot of turbulence of the trapped steam, which results in increase of temperature. Thus turbine will be subjected to excessive thermal overstress, vibration and distortion. So there is a back-up arrangement to trip the generator if it does not trip within 2 seconds i.e. on L.F.P. protection. This is known as Reverse power Protection which acts in two stages. 1st stage reverse power relay operates after 5 seconds time delay and includes stop valve closing/turbine trip. 2nd stage Reverse Power Relay acts after 60 seconds time delay which trips the generator irrespective of either stop valve closing or turbine trip. This acts as a final back up to L.F.P. protection. The power relay designated is 32 G2 and is also WCD type. LOCAL BREAKER BACK-UP PROTECTION (LBB) This is a protection against the main Gen. C.B. failure, which may occur due to (i)
Mechanical failure
(ii)
Trip circuits not healthy.
Hence, this protection acts as a back up to the main generator by tripping all the breakers connected to that particular bus. Relay sensing •
D.C. to the relay extended through trip command (either 86 G or 286 G or B/B protection trip).
•
Over current element senses actual fault persisting.
When both the above conditions are satisfied LBB protection acts with a timer (0.2 secs.) to trip all other breakers connected to that bus. The LBB protection initiates bus bar protection. (Refer to the bus-bar protection scheme). The relay used is designated as 50Z. GENERATOR TRANSFORMER PROTECTIONS G.T. BUCHHOLZ OPERATION Any internal fault in generator transformer will result into rapid increase in the winding temperature resulting in vaporisation of oil (dissociation of oil) accompanied by generation of gas. The generated gas is utilised for relay operation. The relay is a gas-operated device arranged in the pipeline between the transformer tank and separate oil conservator. The vessel is full of oil. It contains two floats b1 and b2 which are to be hinged and to be pressed by their buoyancy against two stops. If gas bubbles are generated in the transformer due to fault, they will rise and KORBA SIMULATOR
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get trapped in the upper part of the relay chamber thereby displacing the oil and lowering the float b1. This sinks and eventually closes an external contact, which operates an alarm.
If the rate of generation of gas is small the lower float b2 is unaffected. When the fault is dangerous and gas production is violent the sudden displacement of oil along with the pipe tilts the float b2 and causes a second contact to be closed and making the trip-circuit and operating the main switches on both HV & LV sides. Gas is not produced until temperature exceeds about 150oC, so momentary overload of transformer does not affect the relay unless the transformer is really hot. Also insufficient oil level in Buchholz relay could lead to same operation. THERMAL OVERLOAD PROTECTION Vapour pressure thermometers or resistance temperature detectors are used for this purpose. The transformer winding temperature and the oil temperature are continuously monitored; when the temperature reaches a certain value it will give indication. Then the load on the transformer is to be reduced. If the temperature rises still further tripping will take place. FIRE PROTECTION Sprinkler system is utilised to protect the transformers from fire hazards. Sprinkler installation comprising of a system of interconnected pipes into which sprinkler heads are fitted on a definite basis of distribution. Sprinkler heads are so constructed that the heat arising from fire will cause them to rupture. Generally the sprinkler/system consists of a compressed air line and a water line. Sprinkler heads are provided in the compressed air line.
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The compressed air line will always be kept in charged condition. When the sprinkler head ruptures, the pressure in the water header will open to send the water into the water header, from his water will be sprinkled on to the transformer. BUS BAR PROTECTION This is a protection against 400 KV bus faults. This protection trips all the feeders connected to the faulted bus zone; it must be reliable and discriminatory so as to (i)
trip only the faulted bus section (zone)
(ii)
not to operate for external faults.
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The current differential senses the fault through high impedance voltage relay (Type FTG) to reduce chances of mal-operation on external faults due to CT saturation. All CTs in that particular zone are parallel with proper polarity to obtain the current differential, which is fed to the relay. Sensing of the particular zone is made through isolator contact status relay (type VAJC). 400 KV yard has 6 bus zones (4 main and 2 transfer zone) as shown in figure below. Section I Bus 1 (A)
X
Bus 3 (D)
Bus 2 (B)
X
Bus 4 (E)
Section II Trans. Bus 1(C)
X
Bus 3 (D)
Each feeder has one common CT for main zone bus protection. The current is switched into appropriate zone (zone A, B, C etc.) through isolator status relay contacts. The operation of the relays is used to energize B/B protection trip buses. The bus bar protection trip D.C. relays (97) are connected to these trip buses again through respective isolator status contacts. In addition to each of the main zones there is an overall check zone to increase reliability of the whole system. This zone covers the whole of 400 KV yard and uses a separate CT core to reduce chances of mal-operation due to CT saturation, loose connection, shorting etc. The A.C current scheme is similar to main zone except it is not routed through any isolator selection. The D.C trip circuit is not complete unless the check protection also operates i.e. for any Bus bar protection trip to occur, both or any one of the main zones and check zone must operate. Also a supervision relay ' 95'connected in the A.C. scheme is provided. This is set at a lower value so that it can sense shorting / opening of one CT circuit current at normal operating value. This provides an alarm and also isolates the B/B protection scheme.
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GENERATOR PROTECTION GT Fire Protection, 30G GT Pressure Release Valve, 30 B GT Buchholz Protection, 30C Bus Bar Zone ‘C’ Protection, 30 D LBB Protection, 30 E Excitation System Fault, 30 Z Generator Diff Protection, 87 GX Gen. Over head feeder Diff Protection, 87 LX UAT A Diff Protection, 87 UTX UAT B Diff Protection, 87UT BX GEN. Over voltage, 59 GC GEN. Inter-Turn Fault, 50 GIX GEN. Pole Slipping protection, 98 GX Stator E/F Protection, 64 GIX GT Restricted E/F Protection, 64 GTX Low Forward Power Protection, 32 G1
UTR-A UTR-B
GT Block Overall Diff Protection 87 GTX GEN. Stator stand-by E/F Protection 64 G2X UT-A Buchholz Trip 30 K UT-A Buchholz Trip 30 N UT-A Fire Protection 30 L UT-B Fire Protection 30 P Gen Negative Sequence Protection 46 T LBB Protection 50 ZX From Relay 86 G UAT-A Back-Up Over current Protection 51 UTAX UAT-B Back-Up Over current Protection 51 UTBX GT Over-Fluxing, 99 GTTX Gen. Back-up Impedance Protection, 21G Gen. Reverse Power Protection 32G Low Forward Power Protection, 32 G1
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GENERATOR PROTECTION Gen. Backup Impedance Protection, 21G Trip Relay 86 G Trip Relay 286 G GT E/F Protection
TRIP RELAY 386 G
GEN CIRCUIT BREAKER TRIP TRANSFER BUS COUPLER BREAKER TRIP
UAT-A WINDING TEMPERATURE 2nd STAGE UAT-A OIL TEMPERATURE 2nd STAGE TRIP RELAY 86 G , TRIP RELAY 286G
6.6 KV UNIT BUA-A TRIP
UAT-B WINDING TEMPERATURE 2nd STAGE UAT-B OIL TEMPERATURE 2nd STAGE TRIP RELAY 86 G, TRIP RELAY 286G
6.6 KV UNIT BUA-B TRIP
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MEASUREMENTS AND CONTROL
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INSTRUMENTS AND MEASUREMENTS Measurement is the source of important and necessary information for any continuous process. Measurement is essential tool and it supports to monitor the operational parameters of any system or the process that performs. Knowledge of measurement is essential for designing any process. In any of the process, the Measurement systems are usually of two types 1) Mechanical and 2) Power type. In making most engineering measurements, we require the assistance of some form of measuring system and measurement by direct comparison (is less general than indirect) or by indirect comparison. Instruments are the measuring means and these are vital exactly like body nerves or the brain in human organs. The basic measuring element or the combination of elements, ' sense'or ' measure'the quantity that sets the limit for the ' integrity' of the measured value. Measuring systems are studied, utilized and designed such that, the output signals truthfully represent the state, condition or characteristic of the system in study. The accuracy of indicated or displayed value may be less if the output signal of the basic measuring element is handled by some intermediate systems. The accuracy with which the condition of a system is controlled by an automatic control system has similar accuracy as measured system must have. Computer control of an industrial process demands that the measurement is fast enough to result in real-time instrumentation and control. Signal processing on output signals bring them into a form or description compatible with the computer. The output signals of the primary measuring systems are converted into proportionate analog electrical signals, and into digital signals. Digital-signal processing techniques are used for developing sophisticated instrumentation systems with multifunction capability. Measuring element provides an output signal that is displayed and recorded in both analog and digital fashion. Digital signals can be transmitted to a distant and remote station, with greater accuracy and integrity than that of analog signals. Electronic data handling systems tackle the signals for display, and recording. Data acquisition systems (DAS) are used for displaying and subsequent operation on the data derived from all the basic measuring systems. The data is transmitted over a long distance; it is reduced and reconstructed to the original form. The data handling operations constitute acquisition, transmission and reduction of the data. Data logging is done by computers, which have provisions for storing data. In analogue signal processing the amplitude is increased or a power through some form of amplification is utilized. In the Figure shown below, various stages of operations carried out on the controls and measurement system has been depicted. Control function is generated after the measured value is compared with the desired or reference value in the form of standard output signals for use in automatic control. Measured data is acquired and pooled at a central location in complex system for display, record or decision-making. Instrumentation system encompasses the entire data handling, starting with the basic function of measurement or releasing the control function signals. Automatic data processing and automatic computation use the logical components in the modern sophisticated systems. The instrumentation system is formed from KORBA SIMULATOR
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application of all the basic measuring systems as well as the processing of the measured data. One has to understand fully the system and select/use instrumentation for processing data.
In power plants, measurements of the parameters viz. the steam pressure, temperature and flow, air & flue gas flow; its quality, and characteristics in combustion chambers (Boilers), the vibration, frequencies, voltages or the current amplitudes of the turbines, and generators etc. are essentially required for keeping proper control/check on operation activities. The chain of devices converts the basic form of input into analogue form or into a digital form that represents the output as known function of the input. The measuring instrument senses, converts and finally presents an analogue or the digital output in the form of display and displacement in a scale or a chart. The first contact that a measuring system has with the quantity to be measured is through the input sample accepted by the detecting element that senses or detects the input signal and then transduces into an analogous form. Acquiring the reliable measurements and then correctly interpreting its meaning invariably leads one nearer & nearer to the desired solution. In making most engineering measurements, we require the assistance of some form of measuring system and measurement by direct comparison (is less general than indirect) or by indirect comparison. Normally Mechanical devices function as Primary Detector/Transducer and the Electrical device serve as a Secondary Transducer.
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MEASURING SYSTEM: INPUT-OUTPUT FUNCTIONING The input/output relationship and the elements have been shown in the functional diagram of measuring systems. Input quantity in the form of sensation is used to drive the primary sensing element as shown, the transducer then converts it in secondary signal, it is then conditioned & converted suitably; additional power source is given for the transducer/primary sensor operation. The signal then becomes the desired data which is transmitted & telemetered then further processed for driving the output instruments like display indicators, recorders CRT displays, limit monitors and other applications. In the subsequent diagram the input-output relationship in instrumentation has been shown. The input quantities are classified as the Desired input, the Interfering input, and the Modifying inputs. These inputs are then amplified/suitably conditioned through amplifier modules as shown. Finally the output components due to interfering input and the output component due to the desired input with modifying components are added, to give adequate output.
MEASURING SYSTEM FUNCTIONAL DIAGRAM
INPUT-OUTPUT FUNCTIONAL DIAGRAM
INSTRUMENTATION AMPLIFIERS Instrumentation Amplifiers are used to boost the amplitude of the signal, buffer the signal, convert the signal current into voltage and separate differential signal, unwanted common mode signal etc The amplifiers are mostly used in the applications of thermocouples, Strain gauge bridges and biological electrodes connected in Wheatstone bridge configurations as shown. The operational amplifiers are intended primarily for amplification of voltage signal derived from transducers’ circuit with accurately adjusted gain values; any variation in gain affects the accuracy of measure of primary quantity. The Instrument amplifiers amplify the low level signals superimposed with common mode voltage and have the characteristics like low drift, high input impendence, high linearity, high CMRR, high noise rejection capability etc.
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The instrument amplifiers are the improved version of differential amplifiers (FET input op-amps. connected in high CMRR using non-inverting configuration), its Net gain; A = (1+2R2/R1) (R4/R3); the potentiometer R1 can give the required gain. Full scale voltage level out of the amplifier is achieved either by sample and hold circuits, analogue multiplexer, A/D converters etc. The Wheatstone bridge is connected with the differential amplifier having voltage Vequivalent and the resistances at the input terminals of the Op-amp become unequal. The bridge is not perfectly balanced and error Vo appears due to Common-Mode input at the amplifier (voltage EBD decides the error) used as instrument amplifier. Differential voltage ediff appears across Rg and creates an imbalance in the current flowing through the transistors Q1 and Q2.The current mirror consisting of Q3 & Q4 forces the imbalance current to flow through Rs.The output voltage is Rs/Rg. The amplifiers are available with programmable gains Basically, the voltage sensitive measuring instruments have very high input impedance so loading must be avoided. All electrical measuring instruments sensitive to current can be used for voltage application (calibrated in voltage scale) by adding suitable resistors in series.
WHEATSTONE BRIDGE CONFIGURATION
INSTRUMENT AMPLIFIERS (DIFF VOLTAGE)
INSTRUMENT TRANSMITTERS/DETECTORS A detector senses the input information, and the transducer puts it in to a more convenient form. The Instrument transducers convert the mechanical input to an analogue electrical output for further processing it in readable/usable form. The sensor or the detector detects the Physical quantities as “signal” by mechanical elastic members or by electrical means. Therefore the transducers require electrical power supply particularly for dynamic measurements, driving and transferring the signal into usable form. The transduction implies energy conversion, and the transducers may be genuine energy converters (called ‘active’ transducers), in some cases they require auxiliary energy source and are therefore energy controllers (called ‘passive’ transducers). The transducer, sensor or detector is the device that measures the physical quantity by electrical or other means. The measuring system consists of a primary sensing KORBA SIMULATOR
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element and a transducer that converts the energy sampled into proportional /corresponding to the input energy signal. Information about the mechanical is usually obtained as displacement (seismic mass converts acceleration to force or movement). A transducer is the first element in the measurement of mechanical quantity (termed as primary sensing element); it requires long-term stability for input-output relationship. A transducer has the size, weight and shape, and it responses to rapid changes in measurand and its electrical output impedance. The transducer/detector elements are classified according to the device or the attachments (Class-1: Detector, Class-2: Detector with Single Transducer and class3: Detector with two transducers) etc. The detectors/ transducers are further classified in two categories as per their functioning; i) ii)
Passive transducers and Active transducers.
Passive transducers depend on material characteristics & physical configuration and the conductance (inverse of resistance), inductances, capacitance etc. so they need external supplies for effecting changes in signal voltage, current or frequency. Active transducers are self-generating and are used for measuring the velocity, temperature, light and force; they normally do not require the auxiliary electrical supplies for driving signal. The active transducers involve conversion of energy from one form to another, and in most cases this conversion operates in both directions. According to signal handling, the transducers are further categorised as: a) The Analogue transducers and b) Digital transducers Analogue transducers provide outputs in analogue form and they need A/D converters for digital application. The outputs of Analogue Transducers express the measured quantity as amplitude that is continuous with time (analogue signal). Digital Transducers (The direct digital transducers) provide digital output signal directly in the form of rectangular pulses of constant duration and amplitude. In the indirect digital type transducers the output signals are sinusoidal with the frequency related to measurand and it works in combination with digital frequency measuring system. The digital encoder for linear and angular displacement is used in direct digital transducers. The digital encoders are either incremental encoder or absolute encoder type. Due to enormous research works, the transducers have been miniaturized too much; these characteristics have led designers to use transducers invariably in all process industries. The digital signals are in the form of pulses, or in the form of sinusoidal time sequence of 1-0 (binary output), but are not essentially dependent on signal amplitude. Digital signal can be transmitted to long distances and it does not get affected due to amplitude change, phase shift (in analogue signal), and it is manipulated without error in electronic processing circuit. Analogue transducers are becoming obsolete presently because of easy refined and cheap technique deployed in digital 1-0 binary system.
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Many quantities e.g. pulse count, determined frequency and encoded position, accurately with high resolution are measured in one or the other form of digital transudation. Frequency outputs include amplitude of frequency converters (voltage oscillators). The digital resonators measure the temperature (even that of very small mass). Periodic time is very easily converted into binary signal so when the frequency of transducer is very high then signal is mixed or multiplied with transparent high frequency signal and there is no difference of frequency. Velocity, temperature, light intensity and force can be transduced with devices that are self-generating and do not require auxiliary electrical power supplies. Measurement of electrical quantity normally does not require transducers; since the primary sensing element is filter or rectifier. The filter networks are common in instruments and filter characteristics are obtained by the use of the feedback of the output signal. Mostly, the electrical elements convert mechanical displacement into voltage thus it normally performs as secondary transducer. Some of the inherent compatibilities of mechno-electrical transducers are as under: 1. Amplification or attenuation may be easily obtained. 2. Mass-inertia effects are minimized. 3. The effects of friction are minimized. 4. Output with sufficient power for control is provided. 5. Remote indication or recording is feasible. In the figures shown below are the transmitters that detect the mechanical forces like the ones given in the mechanical elastic members or the proving rings. The Electric transmitter, shown below, is Foxboro make, electronic force balance transmitter that receives the force through the force bar or the vector flexure. Force is applied via a flexure to the lower end of the force bar, which pivots on the diaphragm seal (It isolates the process from measuring system). Corresponding force bar transmits the effective angle for span adjustment. The force on vector assembly causes movement on the apex. The detector armature (ferrite disc) moves with detector coil and modifies the coupling between the coils, connected to an oscillator circuit and applied to feedback motor to balance the initial disturbing force. This current flowing through the feedback coil is used as the output signal of the transmitter. Vector flexure is used for fine-tuning and larger changes by effective gain change of oscillator as shown here. The D.P. transmitter senses the differential pressure or Pressure signal that is converted into force-balance system for measuring the gauge or absolute pressures as shown in both two figures. Force developed by the primary element is applied via a flexure to one end of the force bar joined together with a thin circular diaphragm serving as flexure. The framework carries the Zero-adjustment spring/ feedback bellows, pneumatic relays etc.
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The force-balance mechanism functions as gauge pressure transmitter, if the lowpressure connection is open to the atmosphere. It also functions as a high-range D.P. transmitter, if both high-and-low-pressure sensing signals are connected to pressure connections of the obstruction elements (discussed in detail in flow measurement section). It can function as a Target flow transmitter, if the primary & secondary elements (forming an integral unit) is particularly used for measuring high viscosity liquids. i.e. asphalt, tar, slurry at pressures up to 100 bars. Similar arrangements can be used as Target flow meters for gas flow. In that case the liquid impinging on the target will be brought to rest so that the pressure increases by V2/2g and the Force on target is given by F=(K. V2.A1)/2. The Force is balanced through the force bar by the air pressure in the bellow so that a 0.2 – 1.0 bar signal proportional to the square root of flow is obtained. Overall accuracy varies from 0.5% and repeatability of 0.1%. The pneumatic system is common for all detectors such as DP , pressure, target flow transmitter etc applications.
ELECTRICAL TRANSMITTER
PNEUMATIC D.P.TRANSMITTER
PNEUMAT. PRESSURE TXR
Capacitive Transducers The Capacitive Transducers work on the principle of variation of capacitance due to cutting of dielectric and change by plate (area) for transducer operation. The Capacitance C = 0.244 KA (N-1)/d, where, K = die-electric constant, A =Plate area (in 2), N = Number of plates, D=separation of plate surface in inch etc. It is evident from above that the capacity effect can be obtained either by change in the dielectric constant or change of area. The change of dielectric constant is used in measurement of level in a container of liquid hydrogen or similar chemical element. The capacitive device detects liquid level even though the reference in dielectric constant between the liquid and vapour state is as low as 0.5. The changing distance between blades of capacitance is undoubtedly more commonly employed for using capacity pickups. The capacitance between the diaphragms & the electrode output is used in a measure of relative position of diaphragms and the distance change between itself and the electrode. KORBA SIMULATOR
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Geometrical variation
Single
Permittivity variation V=dielectric/metal
O/p C
Liquid level gauge Differentia l o/p C± C and C± C
W=dielectric/metal VARIOUS TYPES OF CAPACITIVE SENSORS USING DIFFERENT CHARACTERISTICS
Capacitive Pressure Transducers: Metallic diaphragm/member forms the movable plate of the transducer. The capacitance change due to deformation of the clamped diaphragm can be obtained. Final fractional change in capacitance is given by: C/C0 ={(1-µ 2 ).R4. P}/16.E.dt3 Thus differential pressure can be related with the change in capacitance as above. In a Cylindrical electrode type pressure transducer, the pressure is continuously measured under flowing conditions. The walls of the metallic pipe are used as the outer electrode and a solid cylindrical rod running along the pipe serves as the inner electrode and its capacitance is related by equation: C0= (2 . 0 r.L)/ln (r2/n), Where L= length of central electrode, r2= Inner radius of pipe, ln= natural log, r1=central electrode radius, and r =dielectric constant of the fluid Such transducers are used for indication of pressure under static or slow varying conditions. Bridge circuit is used, the standard capacitor under static & std pressure is kept as reference in the bridge arm. Capacitive Displacement transducer is a standard proximity type sensor in which the movable plate/electrode functions as conductive surface of the object in the vicinity of the fixed plate. Electrostatic instruments (for measuring sufficiently high DC voltage): work on the principle of variable capacity (movable plate), it draws certain amount of energy in the initial stage for setting up the electric field otherwise Electrostatic instruments does not draw any power for AC (rms) measurement; it draws the current however.
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Typical Primary Detector-Transducer Elements & their Operations Type I. Mechanical A. Contacting spindle, pin or finger B. Elastic member 1. Proving ring 2. Bourdon tube 3.Bellows 4.Disphragm 5. Spring C. Mass 1. Seismic mass 2.Pendulum scale 3. Manometer D. Thermal 1. Thermocouple 2. Bi-metal (includes Hg in glass) 3. Temperature-stik E. Hydro-pneumatic 1. Static a) Float b) Hydrometer 2. Dynamic a) Orifice b) Venturi c) Pitot Tube d) Vanes e) Turbines II Electrical A. Resistive 1. Contacting 2. Variable-length conductor 3. Variable area of conductor 4. Variable dimensions of conductor 5. Variable resistivity of conductor B. Inductive 1. Variable coil dimensions 2. Variable air gap 3. Changing core material 4. Changing coil positions 5. Changing core positions 6. Moving coil 7. Moving permanent magnet 8. Moving core C. Capacitive 1. Changing air gap 2. Changing plate areas 3. Changing dielectric D. Electronic, Piezoelectric, E Photoelectric F. Streaming potential
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Operation Displacement to displacement Force to displacement Pressure to displacement Pressure to displacement Pressure to displacement Force to displacement Forcing function displacement Force to displacement Pressure to displacement Temperature to electric current Temperature to displacement Temperature to phase Fluid level to displacement Specific gravity to displacement Velocity to pressure Velocity to pressure Velocity to pressure Velocity to force Linear to angular velocity Displacement to resistance change Displacement to resistance change Displacement to resistance change Strain to resistance change Temp. to resistance change Displacement to inductance change Displacement to inductance change Displacement to inductance change Displacement to inductance change Displacement to inductance change Velocity to inductance change Velocity to inductance change Velocity to inductance change Displacement to capacitance change Displacement to capacitance change Displacement to capacitance change Displacement to current, voltage Light intensity to voltage Flow to voltage
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In view of statistical treatment of data some characteristics of the instruments & related defining terms or terms often applied during working, are given below: Information: The media handled in the process is termed as Information. The information is extracted from extraneous mass input energy by conversion, transudations, filtering process etc. Data is the elemental bits of information in numerical form obtained by experimental means. True or actual value (Va) is the actual magnitude of an input signal to a measuring system. The true value of any physical quantity is obtained by experimental methods as far as possible nearest to the true value; the error of measurement is the difference between the true value and the obtained result. Indicated value (Vi) is the magnitude of raw or directly recorded data by the measuring system. Transfer efficiency (ηT) = Iout / Iin (information) gives the ratio of the input information [Iin] received & output information [Iout] delivered by the pick-up. [Normally ηT < 1,but it is desired to have this value as high as possible.) Correction improves the worth of the result and it is the revision in the form of either an additive factor or a multiplier or both applied to the indicated value. Result (Vr) is obtained by making all known corrections to the indicated value and is derived from Vr = A Vi + B, where A, B are the corrections. Discrepancy is the difference between two indicated values or results determined from a supposedly fixed true value. Error is the actual difference between the true value and the result and is derived from Error = Va – Vr; the value of the error is never really known. Errors can be the systematic or fixed, random or accidental, illegitimate etc and Error is according to the nature and type of activity & techniques used like Human & Experimental Errors, Loading/ judgmental, Variation of conditions, Definition, Blunders or mistakes, Computational, Chaotic etc. Maximum Error is the value by which the result differs from the true value and measured value (associated with small systematic error) and is given by Vr (max or min)] for the true value or the actual value. Certain errors may add directly whereas other may not. A study of error propagation must include consideration of the interrelationship of the various types of error. ei =√ (e1)2 + (e2)2 + (en)2 where ei = overall independent error And e1,en = independent errors.(It’s relationship is often employed for summing equally weighted, independent errors). Accuracy is expressed in percentage based either on the actual scale reading or on full-scale reading: and it is derived from Accuracy = Vr (max or min) – Va.; Percent accuracy (scale) = {Vr (max or min) – VA] / Va} X 100 and
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Percent accuracy (full scale)={Vr (max or min) – Va] / Vfs} X 100,here Vfs is the full scale reading; these are based on scale reading and the full scale readings respectively; The full-scale accuracies have particular settings and scale in use. Precision shows the degree of agreement between repeated results – measurement with small random error. Precise data have small dispersion (spread or scatter), but may be far from the true value. Uncertainty informs possible error or what one thinks may be the range of error. Uncertainty differs from accuracy, for; although accuracy may not actually be known, it is a definite concrete number for a given situation. Uncertainty, on the other hand, is the region in which one believes (or guesses) the error to be. This relation may be described in terms of an analogous idea, a limit dimension. Propagation of uncertainty is the manner in which the uncertainties affect result. Range is the difference between the largest and the smallest result. Scale Range of any instrument is defined as the largest and the smallest reading of the instrument. Frequency range defines the range of frequencies over which measurement may be taken with specified accuracy. Deviation: Deviation or the Residual is the difference between a single result & mean of many results is termed as. Percent standard deviation is the ratio of the standard deviation to the mean. Divisions are marked on a scale; the set of marks or division forms an Index scale and the division moved is the index reading. Linearity: The measurement should response to linear variation (maintain linearity); In order to account for small error in read out system the percentage linearity is maintained as small as possible Percentage linearity is given by: 100 x (maximum deviation of parameter/ Full-scale deviation). Fidelity of any system is defined as the ability of the system to reproduce the output in the same form as the input. Ideally a system should have 100% fidelity and the output appears in the same form as the input. There should not be any distortion. The fidelity refers to the situation that there is no time lag or phase difference between output and input. Response Time: The time required by instrument to settle to its final steady step position after application of the input is defined as the Response Time of an instrument. Whereas the Speed of Response defines the quickness with which a instrument responds to a change in the quantity being measured. Measuring Lag: The delay in the response of the instruments to change in correspondence with the measured quantity is given by Measuring Lag. The measuring lag becomes important where high-speed measurements are required and the time lag is required to be reduced to minimum. Dead time is defined as value before the instrument begins to move after the measured quantity has been changed. Dead Zone is defined as the largest change of
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the quantity being measured to which the instrument does not respond. The Dead zone is also termed as backlash or the Hysteresis. Overshoots: When an input is applied to an instrument, the pointer does not immediately come to rest at its steady state position but moves beyond it or in other words overshoots its steady position. It is desirable to have a little overshoots but an excessive overshoots is undesirable. Steady-state periodic quantity has definite time-cycle whereas the magnitude/time variations of transient quantity do not repeat. Mechanical quantities, in addition to its inherent defining characteristic and distinctive time-amplitude properties, are Static and Dynamic (steady-state & transient). Static system is non-changing in characteristic and is most easily measured. Dynamic inputs require system components, which are sufficiently fast acting to faithfully follow the inputs. Dynamic system refers to the situation when a system does not settle to its equilibrium or steady state condition after the application of the driving force In order to analyse the dynamic behaviour of the system the STEP, RAMP, SINUSOIDAL functions are imposed and results noted for its analysis. Loading: Energy is always taken from the signal source by the measuring system, which means that the information source must always be changed by the act of measurement. This effect is referred as Loading; the smaller is the loading on the signal source of the measuring system, the better is the system. In measuring systems primarily electrical elements sense input & detect the process parameters; the loading of the signal source is almost exclusively a function of the detector. The recorders or the output indicators (being the intermediate modifying devices) receive most of the driving energy from sources other than the signal source, without draining an undue amount of energy from the signal. Measuring Systems: Mechanical Elastic Members and Proving rings Elastic members are the mechanical sensing elements and these are used to change forces into displacements. The Pressure-measuring devices use elastic members of one type or another. The force output from the diaphragm, the bellows, or the Bourdon tube are all based on elastic deformations brought about by the force resulting from pressure summation. The mechanical displacements are usually small in pressure-measuring devices, as with force measurement, (except for the manometer) and so secondary transducers are used for providing interpretable outputs. All instruments available in market make use of properties of either mechanical elastics members or the proving ring characteristics. The driving forces or the movements/functioning of transmitters depend upon the developed force or displacements caused by the mechanical elastic members or proving rings. There are many forms of elastic members for converting mechanical sensation of pressure/flow/level parameters. They fall into one or a combination of following three categories:
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(1) Direct tension or compression (2) Bending moment application (3) Torsion application The Bourdon tube is usually referred as Primary Detector–Transducer for sensing the pressure; it gives output in the form of displacement to drive the linkage chain of the elements. In this case, there is no secondary chain in signal form. But when the displacement from the Bourdon tube is used to move the core of the Differential Transformer for outputting voltage (pressure to displacement and then to voltage in a pressure transducer application), it performs the function of a Secondary Transducer. Secondary Transducer: the ordinary Dial Indicator, in which the spindle acts as a detector through its contact with the signal source functions as Secondary Transducer. Another example of Secondary Transducer is the compression type force measuring load cell. Manometer is an elastic member device whose deflection is proportional to the force. The Torque meters usually, although not always, make use of elastic torsion members. The elastic torsion member twists in proportion to the applied torque, and the deformation is used as a measure of torque. Here again, secondary transducer elements are employed to provide a usable output. Mechanical Proving rings are some form of the mechanical springs & stiff sensing elements and are widely used in instruments as secondary force standards for calibrating testing and weighing machines. Micrometer or dial gages are often used for measuring the deflections; it can accommodate direct tension or compression members. Strain gages are secondary transducers that measure the deflections.
MECHANICAL ELASTIC MEMBERS
MECHANICAL PROVING RINGS
Pressure Measurement KORBA SIMULATOR
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The pressure measurement employs three types/methods as given below: 1. Balancing unknown pressure against pressure produced by a column of known density liquid. 2. Allowing the unknown pressure and measuring the resultant force either directly or indirectly. 3. Allowing the unknown pressure and measuring the resultant stress / strain acting on elastic member of known area Process of balancing a column liquid in U tube of known density; thus gauge pressure = atmospheric pressure + hp and P = hm (A/a+1)(p1-p2) also p = p0 / 1+ (T-T0); =coefficient of cubic expansion. Bourdon Tube: Bourdon tube is a mechanical elastic member as shown above. The simplest form of Bourdon tube comprises a tube of oval cross-section bent into a circle. One end is sealed and attached via an adjustable connection link to the lower end of a pivoted quadrant. The upper part of the quadrant is the toothed segment, which engages in the teeth of the central pinion, which carries the pointer that moves with respect to a fixed scale. Backlash between the quadrant and pinion is minimized by using a delicate hairspring. The other end of the tube is open so that the pressure to be measured can be applied via the block to which it is fixed and which also carries the pressure connection and provides the datum for measurement of the deflection. Bourdon Tube If the internal pressure exceeds the external pressure the shape of the tube changes from oval towards circular with the result that it becomes straighter. The movement of the free end drives the pointer mechanism so that the pointer moves with respect of the scale. If the internal pressure is less than the external pressure, the free end of the tube moves towards the block, causing the pointer to move in the opposite direction. Diaphragm pressure elements: Two basic categories of diaphragm elements comprise of i) stiff metallic diaphragms and ii) slack diaphragms. The unknown pressure is applied to the underside of the diaphragm and resultant movement of the center of the diaphragm is transmitted through a linkage to drive the pointer as in the Bourdon gauge. Bellow elements for pressure measurement: The spring rate or modulus of compression of bellow varies directly modulus of elasticity of the material from which it is formed and proportionally to the third power of the valve thickness. The spring rate of bellow varies directly as the modulus of elasticity of material from which it is formed. Bellow and diaphragm sensors are used for measuring differential pressure as given in the figure below. The bellows are replacing diaphragms since the modulus of the elasticity of the material is better to have good valve thickness.
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The diaphragm elements are made up of corrugated diaphragms with spacing ring stretch welded at central hole. This ensures the safety of the diaphragms when excess pressure is acted upon. Dead weight tester is the simplest technique for determining the pressure by measuring the force i.e. generated when it acts on a known area. The calibration of each instrument confirms the accuracy and correctness of measurement; dead weight tester is one such type of testing and calibration tool. Use of standard pressure gauges in parallel to the gauge to be tested is another method of calibration.
BELLOW TYPE D.P. SENSOR
Flow
DIAPHRAGM TYPE DIFF. PRESSURE SENSOR
Measurement
In flow metering, the Primary method includes the weight or volume tanks, burettes positive displacement category sensors; whereas, the Secondary devices in flow applications e.g. the ventury, flow nozzle, orifices, and variable area meters fall in the obstruction category and total/static pressure & direction of sensing probes fall in velocity probes category. In flow metering, the flowing medium e.g. liquid, gaseous, granular solid and the type of flow e.g. laminar, turbulent, steady state or transient etc. decide the type and size of sensor. In flow measurement, the upstream pressure P1 and the downstream pressure P2 are obtained by suitable tapping provided on either side of the obstruction; the differential pressure P = (P1-P2) is used to obtain the theoretical value of the mean flow velocity V2 =E (2 P/ ) , where E = velocity of approach. In flow measurement, the pressure change is measured and a measure of velocity is obtained. Fluid velocity is measured by reference to aero or hydrodynamic principle. The basic principle of flow sensing using velocity of stream is given by V= (2g h) & the differential pressure is P= h g . Here the, KORBA SIMULATOR
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Flow Relation formulae can be also reffered, i.e; for an incompressible fluid, Flow (Q)=A1V1=A2V2 where Velocity (V)= (2g h) and A=area of cross section; and (P1P2) =V22 2g[1-(A2/A1)2 ] , the Velocity of approach (M)= 1/[1-(A2/A1)2 ; Discharge co-efficient (Cd) = Qactual/Qideal and the Flow-coefficient (K)=CM. Finally the flow is derived by Q = Cd. E. A2 = K.A2 .V2,
Actual volume flow rate is calculated from the mean flow velocity V2, the area of opening offered by obstruction is A2 (= d2/4) and K= flow coefficient (=CdE); and Reynolds Number Rd for a pipe is Rd=D V/µ, in this, D=pipe dia, V= fluid velocity, =mass density, µ= absolute viscosity etc. The discharge coefficients and the flow coefficients are determined experimentally for each size and version of obstruction element and provided by the manufacturers. The Discharge coefficients vary with the flow conditions as determined by the Reynolds number, and the accuracy of the flow rate determination depends on the application of the correct coefficient. Details of the obstruction elements have been given below for proper understanding flow measurements. Obstruction elements e.g. the orifice plate, ventury tube, flow-nozzle etc are used for the measurement of mean flow velocity as also the flow rate. There appears considerable disturbance to the flow pattern and consequent pressure loss due to obstruction. All the measurements involving obstruction elements have their useful range restricted from one-third to the full-scale value of velocity because of the relationship between velocity and differential pressure. The Ventury consists of three parts: entrance cone, throat, and exit as shown and the location of the taps are also shown in the figure. The Ventury is an obstruction element and it creates a total loss of pressure equal to 10-20% of the differential pressure across it. The flow nozzle as shown is a one-piece obstruction element, which can be welded into the pipe for the measurement of high flow velocity of water or steam at high pressure. The Dahl tube is also an obstruction element and it produces longer pressure differential with lower pressure loss as compared to the venturi. The Dahl tube given in figure consists of a short length of parallel lead in pipe followed by the converging upstream cone and the diverging upstream cone. A circumferential gap is kept between the two cones and the down stream pressure is tapped at the location of the gap. The Orifice plate is the simplest and cheapest obstruction element, with its’ coefficient of discharge being the lowest at 0.6. Orifice is a thin metallic disc arranged concentric with the pipe in most cases. It may be eccentrically located when intended for use with fluids containing small traces of particulate matter. The segmental orifice plate has a hole that is partly circular located below its centre. The three types are shown in figure. The edge of the orifice plate on the upstream face should be sharp as rounding or burring faces considerably affect the flow rate. The dimensions
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of one of the sharp-edged versions are shown in figure. The size of the opening is designed so as to produce approximately the maximum pressure difference at the maximum, rate of flow. Depending on fluid type its low rate, the orifices are made of stainless steel, nickel, gunmetal, ebonite or plastics. Pitot tube named/derived from its inventor, Henry Pitot, is used for measuring flow since in a horizontal pipeline, flowing fluid coincide with velocity vector which varies from zero at the wall and maximum at the centre. The Pitot tube is having a small opening facing the fluid flow direction. The tube is used to determine pressures resulting from total flow-rate rather than change of rate and it determines the impact pressure. Pitot tubes are also used for gas flow measurements. The Pitot tube is combined with static opening. Proper alignment of the tube with the flow direction and YAW-ANGLE (probe axis and the flow stream line at the pressure opening) of Pitot tubes need to be properly aligned with flow direction and the Yaw – angle should be zero. The pressure built up in the Pitot tube is higher than the free stream (static) pressure. The excess pressure h is termed as impact pressure. Excess pressure P2P1 = p = g h and velocity at Pitot tube mouth is V1 = (2g h); finally V1 = (2hm m g/ ) ,where hm = diff. in levels of the manometer m = mass density of the liquid, = mass density of fluid in motion. Figure below show, the obstruction type flow sensors and their flow pattern.
FLOW METERING SENSORS AND DEVICES
FLOW CHARACTERISTICS
Flow Measurements using Rota meters: The Rotameter flow system is versatile in that it can be designed to measure the flow rates of liquids widely ranging in their viscosities and volume flow rates as low as 0.1 KORBA SIMULATOR
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cc/min. The mass flow rate, however, is made totally independent of density, by making the density of the float double that of the fluid. The float is made hollow or of solid light plastic material. To make the float independent of viscosity drag, the length of the float is made small while ensuring that the Reynolds number of the flow is not greater than 2000. In the Rota meter float systems, the tube is made of borosilicate. The guiding spindle or shaft carrying the float at the lower end passes through upper part of the tube, with upper end serving as the index on the scale. Rotameters are designed on the basis of the types (two) of flow: i) The vertical flow
ii) The horizontal flow.
Vertical flow type Rota meter-float instruments are of constant head variable area type flow meters /rotors. The Rota meter-float system employs a float kept in a fluid stream so that its’ position is a measure of the velocity of the fluid. The measuring fluid is passed through a tapered glass tube as shown in figure. The float obstructs the flow and the fluid flows through the annular clearance between the float and the inner wall of the tube. The float comes to rest, when the forces working on the float due to upward fluid flow, is balanced by the weight and buoyancy forces. The clearance between the float and the tapered glass varies in area with the position of the float, and it sets up a differential pressure across the top and bottom surfaces of the float. The float always assumes a position for each velocity holding the differential pressure constant. The position of the float is always hold the D.P and thus is calibrated in terms of the fluid velocity. The flow rate and differential pressure are given by: Flow rate Q=ACd = {2 Vfg ( f – )}/Af , and the Differential pressure P = (Pb -Pt) = {2 Vfg ( f – )}/Af ;here A is area; Cd is the discharge coefficient. In Horizontal flow type Rota meter-float systems; the tube is in position and the float is backed by spring as shown. The spring force to the displacement of the float and is not a constant. Hence the nonlinearly related to displacement. The secondary transducer and circuitry are adjusted to correct this non-linearity and the final signal to the velocity.
the horizontal is proportional flow rates are the associated is proportional
Turbine Type Flow Meters as shown has axially mounted freely rotating turbine rotor; its axis coincide with pipe centreline and flow direction. The fluid in motion impinges on the rotor blades, and torque is developed on each blade/wing. The rotor rotates in proportion to the fluid velocity. At steady state, the flow rate is proportional to the angular velocity .The rotor is hinged rigidly inside the meter. Electromagnetic transducer nowadays measures the speed of the rotor and associated digital read out is obtained as against the older methods of using traditional counters with gearing mechanisms.
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HORIZONTAL FLOW ROTAMETER
VERTICAL FLOW ROTAMETER
Magnetic Flow Detector: When the conducting liquid flows through a magnetic field an e.m.f is generated which is proportional to the rate of flow of the liquid; the e.m.f. is used to drive an indicating or controlling equipment. This type of sensor is able to overcome the inaccuracies caused due to pressure drop and any restriction in flow of slurries that may clog and no change materially with time. Basic advantages of magnetic flow meters (although resistivity is limited in certain applications) are the density variation, not immune to fluid viscosity, no affect due to suspended solids, bi-directional flow sensing and fast response inductance of the coils, this waveform can not be entirely achieved Electromagnetic flow Detector: It is based on Faradays Laws of electromagnetic induction. According to this when conducting fluid passes through a pipe of nonconducting and non-magnetic material it can be treated as equivalent to a set of the moving parallel straight conductors lying in a plane perpendicular to direction of motion. EMF E = BDV where B = magnetic flux density, Wb/m2 D = pipe diameter in mtr, V = velocity of flow, m/s. Maximum EMF is induced when the electrodes placed across the diameter of the pipe and the direction of flow makes the fluid medium remain in continuous contact with the element between the electrodes; equivalent shortcircuiting effect is observed and the actual voltage across the electrodes becomes less than B.D.C. Such type of flow meters do not obstruct the flow and can measure the velocity of flow of slurry and corrosive liquids and are insensitive to the viscosity, density and temperature. Electromagnetic flow meters are suitable for measuring a wide variety of liquids such as dirty liquids, pastes, acids, slurries, alkali, etc. Temperature, pressure, density, viscosity, conductivity etc. have no bad effects.
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In the figure of the waveform as shown below, the square wave excitation is used; D.C supply to the coil is switched on/off at 2.5 Hz frequency with polarity revers al every cycle. At a) Shows the ideal current waveform for pulsed D.C excitation but because of the inductance of the coils, this waveform can not be entirely achieved . at (b) Shows the excitation from constant current source. and at (c) the signal produced at the measuring electrode; signal is sampled at five points during each measuring cycle and true value of flow is achieved.
MAGNETIC FLOW DET
ELECTRO MAG. FOW DET
WAVEFORM IN THE COIL
Doppler Effect & Ultrasonic Flow Detectors According to Doppler effect, the frequency of sound changes (this indicates the speed) if its source/reflector moves relative to the listener or the monitor. Two piezo electric crystals (one a transmitter and the other a receiver) are potted/fitted on pipe wall as shown in RHS bottom. Velocity is proportional to the frequency and thus V=C (F2F1)/(2F1 COS ). In this type, the flow stream must have some discontinuities and pipeline be acoustically transmissive. In Ultrasonic system the time of flight of the sound wave between the two points get modified by the velocity of flowing medium and difference between the flight time is to be directly proportional to the flow velocity.
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Ultrasonic Flow Sensor
Ultrasonic Sensors mounting in pipe
Measurement of Level systems The practical method of knowing the level of contents (volume); presence of substances, the level sensors are needed for detecting these levels and provide proportional read out of the level with respect of chosen datum. Position of the sensor head is such that the problems of turbulence created by the contents flowing in and out to the tank is subsided. Corrosion effects caused by the components of the sensing system, high temperatures, abrasion in granular materials may cause undue friction and change the mass of floats and level affected. Level measurements, associate with possible errors of i) surface tension effects in the sensing media, ii) turbulence occurring at the sensor due to material flow, iii) changes of the mass of float due to sediment built upon the float sensors, and iv) temperature and pressure changes to the contents. Level Measuring systems are generally of following types: 1.Side gauges 2.Float driven instruments, up-thrust buoyancy 3.Capacitance type probes 4.Pressure sensing 5.Microwave and ultrasonic time transit method Levels in tanks are measured by using pressure of column of liquid at constant density and is obtained as gh; gh = ( mhm – y)g; m = density of mercury The mercury levels in the two legs of the manometer are adjusted initially to be at the same height as the bottom of the container, then h = K hm Normally, a condensing chamber is provided at the top of the second tube of the manometer and is filled with the same liquid as in the container. Level sensing by Air bubbler system: In this method air pressure, built up due to admission of air through a tube into the container of a liquid, is measured. The air is supplied past a regulating valve so that it just escapes from the bottom end of the tube as bubbles. Then the pressure in the tube equals the pressure due to the head of liquid, h, above the lower end of the tube, and is given by P = h g.
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Level measuring system type using Horizontal U-tube/manometer: In this type the weight of the tube and its liquid contents are measured by any weight or force measuring transducer, by using null-balance principle. The level measuring system as shown gives h= h1 – hm ( m/ -1) and h = K hm as h1 is held constant. Alternatively, any other transducer such as the diaphragm type can be used in place of the manometer for sensing the pressure difference between the top and bottom of the closed container, while retaining the condenser. If the liquid is of constant density, then the weight of the closed container is obtained by means of load cells. Using containers of uniform cross-section, the level of the liquid can be ascertained from the weight. Level measuring system using Hydro pneumatic devices: These are based on the ordinary float idea, or that of the hydrometer. The simple float is used primarily as a liquid-level detector and makes no allowance for change in density of the supporting liquid, it being assumed that the float is always immersed to the same depth. On the other hand, the hydrometer uses the depth of immersion as a means for detecting variations in specific gravity of the supporting liquid. Both these level sensors of course, function at static conditions. Level instruments using magnetic sensors are installed in the tanks needing tripping, alarming and lamp indications for detecting high/low level states of the liquids in tanks. Magnetic indicators (as shown in the figure) use floats to follow the liquid surface using mechanical linkages for operating remotely located read out devices. Level Sensing By Hydra-Step: The Hydra step is self-validating by the continuous comparison between adjacent channels type remote display unit. It is used in place of direct gauge glass for level measurement of boiler drum. This is also accepted as replacement of direct gauge-glass by Boiler Inspectors. It is observed that approximately 15 seconds are only available for tripping down the boilers in cases when level of the high capacity (200MW or more capacity) boiler drums reach beyond its normal operating levels, thus, the Hydra step is the fastest gauge glass to support operating people for safe operation. Necessary precautions are taken to maintain the security of the hydra step such that plus minus 1 step tolerance is eliminated in acknowledging the drum level in the gauge glasses. Schmitt trigger circuit is utilized for creating logic matrix and relaying. The Hydra step operates on the philosophy of continuous comparison between adjacent channels by the logic matrix and it ensures that the indication presented to the operator has been fully verified. The difference of standard resistivity is sensed and indicated as steam (red colour, at higher resistivity) and as water (green colour, at lower resistivity) through logic matrix obtained by Hydra step cells which are mounted at the pre-decided locations of the gauge glass unit. The measurable resistivity is prominent at 100 bar to 183 bar or more pressures. At this pressure and sufficient temperature (200 to 360O C) the resistivity of water is above one mega ohm and of steam is 100-mega ohm. The cells are energized at 10 volt 15Hz and 10 micro amps. The voltage drops from 4.9 volt to 2.9 volt as water level increases. Level Sensing by Constant Head Unit/Chamber: Almost all type of transmitters used for level sensing of high temperature/pressure vessels, are provided with a constant head chamber/unit in the sensing/impulse lines of the transmitters. The CHU KORBA SIMULATOR
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ensures the connectivity of impulse lines at normal room temperature. When the transmitter is charged for the first time, the impulse lines of a heated vessel/CHU gets filled with air and condensate liquid. CHU is vented for a few minutes. The air gets vented after a few minutes of charging and the condensate liquid remains in CHU that comes in contact with atmospheric temperature which after an hour or so, gets cooled to atmospheric temperature thus by this time the impulse line connecting the transmitter has attained the normal room temperature. The process of venting is adopted for flow/pressure/level applications also in order to keep the input connections to the transmitters cool. Mounting arrangement is shown below.
MAGNETIC FLOAT LEVEL SENSOR
HYDRASTEP LEVEL INDICATOR
CONSTANT HEAD UNIT CONNECTION
MERCURY FILLED U TUBE LEVEL SENSOR
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Photovoltaic or barrier cell is a semi conductor device in which proper processing produces a insulating barrier layer between the semiconductor and the metal layer T. If light is incident on the barrier layer, an e.m.f is developed across the base plate and the top metal layer, with the base plate being the positive terminal. A thin film of gold or platinum is deposited on crystalline selenium with a base plate of iron. Presently, cells with silicon and germanium for semiconductor layers are available. Since all the energy of the current is derived from light source and the cell acts as an energy converter, the current, though of measurable magnitude, represents very small power. The short circuit current is proportional to the area of the cell and increases linearly with luminance as shown. The sensitivity of cell is of the order of 1 mA/lm. The open circuit voltage increases in approximately logarithmic fashion, as shown and is independent of the cell area. When the incident light flux is increased by 1000 times, the resistance of the cell falls by 100 times. The cell is considered as a source of current and used with external circuits of very lower resistance. If amplification is desired, amplifiers with very low input impedance are used. The spectral response of the selenium is almost similar to that of the human eye and extends from about 250 nm to 750 nm, with a maximum response at about 570 nm. The response of the germanium cell is primarily in the infrared region responding to wavelengths from 250 nm (near ultraviolet) to 2000 nm, with a maximum sensitivity around 1500 nm. The silicon cell has its threshold wavelength at 1200 nm. All the above cells are also sensitive to X-rays, and rays and gamma radiation. Photo Electric Transducers: The P. E. Transducers cell behave as a light control variable so the output is obtained which is proportional to the intensity of the light source. The Photocells find its application in measurement as a strain gage, dew point control, edge and tension controls etc and in mechanical measurements including simple counting where the interruption of beam of light could be implied. Light Sensitive Detectors, Photo Sensors or Photo Cells are some other forms of Photo Electric Transducers category of instruments. The older version of Electronic type photocells consists of a cathode/anode combination within an evacuated glass or quartz envelope; light impingement freezes electrons to flow thereby causing a flow of small current. The photoconductive cells consist of thin film of material such selenium, metallic sulphide/germanium coated between electrodes on a glass plate. In the figure below the constructional feature of photocell has been shown, as to how light energy impinges on the cells, the base plate, the semiconductor layer and the emf.output is derived in a photocell. The curve shows the curve of light flux versus emf, relative spectral sensitivity versus wavelength ( ) and the light flux versus the short circuit current Io at impedance loads as shown.
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PHOTOCELLS ‘ CHARACTERISTICS
LIGHT FLUX VERSUS Eo and Io
Load Cells: The Load Cells are basically the strain gauges. Load cell measures the deformation produced by the force or the weight. Load cell consists of a short column or strut with strain gauge attached thereto and the force reflects of strains the block for sensing the input, In this the load is converted into deflection analogous to the weight or force. It yields a measure of the quantity in hopper or feed of dry or liquid materials; forces up to 5 MN can be measured. An accelerometer works by the inertia of a concentrated mass; this serves to measure the characteristics of dynamic motion, i.e., displacement, velocity, acceleration, and frequency, through application of Newton’s laws of motion. The pendulum, or any simple mechanically vibrating member, may serve as a time or frequency transducer, chopping the passage of time into discrete bits. A Load cell converts force into movement against the reaction of a spring. The movement is then measured by a displacement transducer and converted into electrical form. Combined actuator transducer (CAT) is used for automatic optical instruments having a torque motor and a miniature preamplifier. In Electronic Force Balance system the displacement is caused by applied force and is sensed by displacement transducer; its output is fed to servo amplifier to give an output current through restoring coil. Various forms of load cell, Vibration pickup, and combined actuator transducer have been drawn in the figures below.
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COMBINED ACTUATOR TRANSDUCER
LOAD CELL AND ITS CIRCUIT
VIBRATION PICKUP
Hall generators and Hall effect sensors: Hall Effect Sensor mainly comprises of Hall generator. The electrical characteristics as per Hall (scientist) depend upon the material chemistry and features like Hall mobility, drift mobility and the relationship of drift and microscopic mobility to conductivity. If a magnetic field ‘B’ is applied to a flat or round conductor, which is carrying input current ‘Ic’ in the direction perpendicular to Ic, a Potential difference VH proportional to the applied magnetic field B appears in the direction perpendicular to both Ic and B. The potential difference VH = K. IC. B Hall generator is a four terminal solid-state device and its’ output voltage is proportional to the normal magnetic field and magnitude of input current I in. As the conversion takes place of input or load current to the output voltage (Hall Voltage) in magnetic field generated by the input current around the current line, it provides total isolation between sensing circuit and the current line being sensed. In electrical engineering applications Hall output Eh= K . IC . If . e (off set) Where K = product sensitivity in 9 mv/ A), IC = control current, If = load current (Amp) and e (offset) = zero current (off set 9 mv) at If = 0 If Ic is kept constant Eh (hall output) is linearly proportional to the current being sensed. Hall effect sensor contains four building blocks viz. hall generator, the magnetic core, the amplifier and temperature compensated constant current source. Output of the Hall sensor does not appear as a current but it appears as a voltage or electromotive force generated by magnetic field. The output voltage of the Hall effect device is proportional to the control current ‘Ic’ or ‘Iin’, therefore it is necessary to operate Hall current sensor by constant current regulation. AC/D.C flux are sensed with no limitations by Hall sensors.
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HALL EFFECT DEVICES
Optical Pyrometer: It works on the principle of Thermal radiation. Such detector avoids physical contact of the system at more than 1400OC temperature, thermometer elements get damaged / distorted (rather not possible to be mounted). Instrument based on measurement of radiant energy emitted by the hot body are developed and used to estimate the temperature of the body these are known as “Pyrometers” also. Radiation is classified into several regions / bands depending on its characteristics and wavelength. The radiation detectors are thin strip of blackened metal foil of platinum and usually called bolo meter, a change in the resistance of the foil is indicative of the temperature of the hot body. Energy levels of 3x10-8 w can be detected; the time constant is 3 msec. approx. Total radiation pyrometer follows the Stefen-Boltzman’s law Et= R T4 Where T= temp. in 0 K R= Stefan’s Const. Photo Electric Transducers:It covers, Light Sensitive Detectors, Photo Sensors or Photo Cells. The Electronic type transducer consists of combined cathode/anode within an evacuated glass or quartz envelope; light impingement freezes electrons to flow thereby causing a flow of small current. The photoconductive cells consist of thin film of material such as selenium, metallic sulphide/ germanium coated between electrodes on a glass plate.
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OPTICAL PYROMETER
PHOTO ELECTRIC TRANSDUCER
Piezo Electric Transducers: Certain materials possess the ability to generate an electrical potential when subjected to mechanical strength or dimension changes when subjected to voltage. This effect is known as Piezo Electric Effect. Quartz is undoubtedly the most stable Piezo-Electric substance and although its output is quite low but it is used almost universally in electronic oscillators. The Quartz is ground to the shape of a rectangular or square plate and firmly held between two electrodes contacting its faces. Piezo Electric Transducers are used for force & pressure, temperature and acceleration etc. measurements. Forces from 1N to 200 KN with linearity approaching =-1%, outputs sensitivity of 125 mV per k Pa for 2.5 mm thick and 10 cm2 area of the crystal are available. Acceleration transducer may be about 4 mm dia and 10 mm long having 2 gram weight that can operate up to 2000C. Crystal also gives very linear & sensitive correspondence between resonant frequency and temperature and allows measurement of absolute and differential temperature. The effect is in crystals; the deformation causes the displacement of internal charges and equal opposite charge on opposite side of the crystal. Potential difference Vo=Q/CP. Mechanical resonance and mounting orientation decides the frequency limits. Range 2 Hz –1MHz, temp. 3500c.
PIEZO ELECTRIC SUBSTANCE
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Radioactive gauges Radioactive vacuum gauge is based on the effect of ionization of the gas, whose pressure is under measurement by -rays emitted from a radioactive source kept inside the gauge. The valves are of sufficient thickness and of a material, which does not allow radiation leakage. The number of ions formed is proportional to the gas pressure as long as the range of the -rays exceeds the dimensions of the chamber. The ion current does not increase any further when the -particles emitted are totally absorbed by the gas inside. The ion current is proportional to the pressure and the range pressure is the largest with this gauge. These gauges can be used from 10-4 torr to 10-3 torr. In the Radioactive thickness gauge (shielded source type) shielded source of nuclear radiation and radiation detector is used as the source of radiation is shielded except in the direction required for absorption or penetration through the material whose thickness is under measurement. In the Radioactive thickness gauge (Back-scattered detector type), the amount of back-scattered radiation from the test sheet and the backing material (different atomic number) depends on the amount of scattering and absorption of radiation in the test sheet & the backing material and varies with the test sheet thickness. Gamma and X-rays are highly penetrating and are used for heavy metals and thick specimens. Beta particles are much less penetrating and hence suitable for measurements of thickness of metallic foils and thin deposits of metals on paper, rubber or plastics. The -rays are used only for very think foils of a few microns thickness. Gamma and X-rays are normally composed of radiation of different wavelengths and hence thickness gauges have to be calibrated for each radiation and for each material. The thickness gauges are fast and need not be calibrated everyday. No mechanical contact between the gauge and test piece is necessary. Radioactive Level Gauge has column of liquid, the radioactive source & detector. Ionisation Transducers: Ionisation Transducer consists of a glass envelope with two internal electrodes filled by gas or gas under reduced pressure. The circuit shown indicates the radio frequency power source ionizes the gas in the transducer in the field from two external electrodes Space charges created and DC output signal furnishes the potential of the electrode. Either C1 or C2 varies to balance the electric field and produce an output. The length of the path determines the intensity of radiation received by the detector. As is the case with sheet materials, the characteristic is non-linear. Where linearity is required, a good number of strip sources and strip detectors may be located along the sides but on opposite faces so that the total output of the detectors when added makes up the output signal.
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RADIOACTIVE VACUUM GAUGEs
RADIO ACTIVE DETECTOR (scatter)
RADIOACTIVE THICKNESS GAUGE
IONISATION TYPE SENSORS
ELECTRICAL SENSORS AND SYSTEMS IN MEASUREMENT: The electrical element transforms the analogue displacement into Voltage or the current through the passive elements i.e. resistors, inductors, capacitors. The variable resistance (e.g. ordinary switch, sliding contact/potentiometer, wire resistance strain gauge, thermisters, thermocouple etc.) or The variable inductance (single/double coil self inductive, simple two/three coil mutual inductive, variable reluctance in moving iron/moving coil/moving magnet etc.) effect changes in transducer applications. Variation of permeance of magnetic circuit causes a change in the flux and voltage is developed due to expanding or collapsing of the flux. The variable reluctance is generated when the magnetic lines of flux emanated from the permanent magnets of the systems, are cut by the turns of the coil, this principle is used for transducer operation and thus the relative motion is incorporated into the device. The active pick-ups e.g. tacho-generator, thermocouple, photovoltaic diode and piezoelectric crystal, produce voltage outputs related to non-electrical inputs, without
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the need for separate voltage supplies and transducer bridges. However, more often than not, they are coupled to instrumentation amplifiers, which do require power supplies. The piezoelectric substances, and the thermocouples with thermoelectric characteristics fall in active sensor category. Digital Tachometers, Counters & Electronic Frequency Meters Tachometers produce voltage & frequency proportional to the shaft speed; one type uses the voltage level and the other one-frequency waveform. The output may be single or three phase (preferred for lower speeds, in which effect of ripple is minimized, it reduces oscillation at lower speed). Signal is rectified & smoothened in case of voltage level type tachometers. In such case the system is very dependent on the impedances of the components in the circuit so has poor accuracy. The toothed wheel and magnetic probe which considers the frequency level type, used universally nowadays by using disk of non-magnetic material that eliminates the influence of shaft magnetization. The voltage type was normally working on analogue signals generation principle. In Digital tachometers, the shape of the output signal is similar to the shape of the pulses those are counted over a given time and the time between the pulses is measured. In linear function tachometer, Pulse counting mechanism is employed to give digital outputs for converting the rotational speed. The capacitor system is excited by a high frequency source so that the output is effectively a pulse-modulated signal. The incremental encoders are used for the measurement of either linear velocity over limited distances or rotational speeds. Pulses are generated due to the capacitance change between a probe plate and serrated rotor; its rate is proportional to rotor speed. The pulse counting method provides measurement of the average speed only and the accuracy depends on the clock period and the resolution varying with speed, so very low speed measurement is not possible The clock provides the pulses to open the gate for the prescribed period to set the counter before each count and simultaneously update the digital output. Sensing the direction of motion and indication of the same is possible. In case of inverse function tachometer the clock each incremental pulse of encoder gates pulses. Control logic is used to reset the counter and update the output at each pulse from the incremental and encoder disc. The instantaneous speed of rotation at any instant during revolution is reckoned from the counter and the transient in speed are detected. Electro Mechanical Transducers (Tachometers) Angular speed of rotating shaft measurement is very important particularly in the automatic speed control system. The tachometers are used for analogue indication of speed and DC and AC speedometers using voltmeters. The dc tachometers converts shaft rotation (proportional to speed) signal into an electrical signal. Permanent magnet stator and wound rotor principle is used for dc generator (tachometer). The voltage relationship is given by:
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E.M.F, Eg = (NPΦ ΦΩ)/a ; where N = total number of conductors of the armature P = number of field poles Φ = total magnetic flux per pole, Wb Ω = speed of rotation, rad/s a = No. of parallel paths in armature winding For desired high resolution of the order of ±0.1% to ±0.01%, feedback- using the dc tacho-generator or the ac pulse-generating tachometer type measuring system is used. In eddy current drag-cup type tachometer, the eddy current is set up in the cup that interacts through the magnetic field in a manner as to follow the magnet and therefore torque developed is proportional to the magnet and cup. Thus the angular deflection is proportional to the angular speed of rotation in steady state condition.
COUNTERS TYPE FREQUENCY METER
ELECTROMECHANICAL TACHOMETERS
Linear Variable Differential Transformer (LVDT): is basically a Differential Transformer that uses the principle of variable inductance. LVDT provides an output a-c voltage proportional to the displacement of a core passing through the windings enclosures. LVDT is a mutual inductance device that generally makes use of three coils. The core displacement results in a proportional output on either side of the null position within limits. In general, the linear range is primarily dependent on the length of the secondary coils. While the output voltage magnitudes are ideally the same for equal core displacements on either wide of null balance, the phase relation existing between power source and output changes 1800 through null. It is therefore possible to distinguish between outputs resulting from displacements on each side of null, through phase determination or by use of phase-sensitive circuit arrangement. The core movement (length change) causes the inductance variations. For stability and performance improvement, addition of balanced, differential, phase sensitive detection systems are used. The input voltage of LVDT is limited by the current-carrying ability of the primary coil. The sensitivity increases with increased
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number of turns on the coils in a LVDT. There is a limit, however, determined by the solenoid effect on the core. When utmost sensitivity is desired and the transformer output is amplified, it may be necessary to use external circuitry to improve the null balance condition Exciting frequency, sometimes referred to as carrier frequency, limits the dynamic response of a transformer. The LVDT sensitivity is directly proportional to exciting voltage and, as indicated above, also it increases with frequency. The LVDT is used in primary detector to convert mechanical force into a proportional electrical voltage. It is also relatively insensitive to high or low temperatures or to temperature changes and it provides comparatively high output, often usable without intermediate amplification. It is reusable and the cost is also reasonable. A voltage dividing potentiometer of sufficiently high resistance is placed across the transformer secondary output to avoid appreciable loading. In the figures below the process of inductance change due to core movement, the core movement causing the differential transformation, the force balance type LVDT sensing, and complete detector using LVDT have been shown.
FORCE BALANCE TYPE LVDT
LVDT SENSING, DIFF. Xter, INST DETECTOR
LVDT FUNCTIONING
Permanent magnet moving coil type measuring system makes use of the driving force/the torque generated by the D.C current flowing in the coil. Such meters are also used for measuring ac current and of frequency higher than 150 Hz as well as dc current. The electrometer amplifiers are used for extremely low current measurements, by use of Transfer Standard in which a thermocouple and a small KORBA SIMULATOR
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heater coil enclosed in an evacuated glass bulb, is used. Bridge rectifier circuit is used for measuring the Average value of current) by unbalance current flow through a permanent magnet moving coil instrument. Measurement and displays of Temperature Systems In simple thermometer, or the bimetal strip thermometer the Temperature detection is based on differential expansion of two different materials. Hot Wire Resistance Transducers use transfer of heat through conduction by using resistive element in physical contact with the system. The Resistance thermometer and the thermocouple are categorised as the secondary transducers. The temperature is determined in thermocouples from thermoelectric properties of materials or combinations of materials; whereas the variation of resistance gives the temperature figures of substances from resistance thermometers detectors. Potentiometers consist of resistance element provided with a movable pot/contact, in translation or rotation or combination or helical motion in multi-turn rotation. The resistance element is driven by a.c or d.c. excitation and the output voltage is ideally linear function of the input displacement. The Potentiometer network is used for measuring very precise & accurate value of voltage; small e.m.f is obtained from active transducers (Thermocouple etc.); this e.m.f. is compared with standard cell configured in detection of unbalance current through slide wire and the potentiometer resistance coil. Potentiometer N/W are used for Commercial versions of slow varying transients & faster voltages/current in applications like range-change, standardization and read out of unknown voltage through recording instrument. Potentiometers are available with varying design & provisions. Manual balancing is possible by galvanometer. Automatic balancing of the potentiometer requires high gain amplifier and a servomotor. Nowadays the Zener diode reference is used to provide the voltage for standardization in place of standard cells. The circuit using potentiometer has been shown in the figure below. Resistance Temperature Detector (RTD) is a resistance transducer that changes its value due to change in temperature at the sensing source. RTD is a wire element that is wound on a firmer, (in form of a coil) and in a sheath or the protecting tube to achieve small size and improve thermal conductivity. The information required for selection of the RTD sensors is; the sensitivity of the RTD, response (slow or fast) time (0.5 – 5 sec) giving the thermal conductivity, good/poor thermal contact of sensor with the medium etc. Platinum-metal sensor for use as RTD, is quite sensitive (having 0.004/0C sensitivity), easily repeatable but is certainly costlier than the Nickel sensor (having 0.005/0C sensitivity). The Platinum resistance temperature detectors (RTD) enable achievement of very high resolution of the order of ± 0.0001k and high sensitivity. The variation of platinum resistance is too much, as is clear from the fact that the resistance of value 7000 Ω at 2000 K can fall down to even 6 Ω at 6 K.
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Linear fractional change in resistance and resistance versus temperature values decide the sensitivity of the RTD. Since resistance is given by Ro= ρ0.l/A, so, any of these three items/figures (i.e. specific resistance, length, area.) can be varied for obtaining resistance change of the RTD.Also, RT = Ro (1+α0∆T); new value of RT gets changed in correspondence with temperature due to variation of initial resistance (Ro), difference of temperature (∆T), temperature co-efficient (α0). The increase/decrease of resistance is linear. Tables of RTD resistances to corresponding temperatures are available widely for calibration, testing and confirmation of resistance versus temperatures. In the given circuit, the RTD is connected in a bridge circuit in which a null condition is detected by a galvanometer or an amplifier for driving an indicator or the recorder. A compensation line in the R3 leg of the bridge is required when the lead length is long enough.
RESISTANCE TEMPERATURE DETECTOR
POTENTIOMETERIC RECORDER CIRCUIT
Thermocouple Temperature sensor produces an e.m.f that is proportional to the temperature of the junction of two dissimilar metals. A thermocouple is a junction of two dissimilar metal wires usually joined to a third metal wire. The e.m.f. is almost linear and repeatable with change in temperature. Many metals and alloys exhibit the thermal electric effect but only a few metals are used for temperature sensors. The generated voltage is fed to operational amplifiers of high input impedance to avoid loading. The amplifier output (Vout) is used in indicators, controllers or recorders. Ice bath is generally used, for making the reference junction. Otherwise reference junction is at triple point of water apparatus, kept at 0.01± 0.0005OC temperature. All thermocouples are encapsulated by stainless steel sheath or hard brass/copper tubes and are protected from contamination and mechanical strengths. Most of the thermocouples can be used in oxidizing environment up to 7500C and reducing environment up to 10000C. The quality of wire produced from batch to batch may vary slightly with the result that the e.m.f produced may not exactly conform to the values made available from standard tables. In all cases, the polarity of the thermocouple material mentioned first is positive for temperatures greater than the reference junction temperature.
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Platinum metal is stable and platinum-rhodium thermocouple is invariably used, as primary standard for temp between 630.50C and 1063OC.Its sensitivity is only about 6 µV/0C, and is used up to 15000C. Constantan (Ni 40%, Cu 60%) is another alloy that is used with copper, iron or chromel (Ni 90%, Cr 10%). Copper constantan T/C has the maximum sensitivity of 6µV/0C and is useful for the range from -20000C to +4000C. Iron/constantan thermocouple is most widely used for industrial applications for the range of temperatures from -1500C to + 10000C. Chromel Alumel (Ni 94%, Mn 3%, Al 2%, Si 1%) is another widely used thermocouple for temperatures from -2000C to +13000C, and is preferred for use in non-reducing environment at temperatures between 7000C and 13000C. The circuit below shows connection of T/C in Op-amp input through cold junction compensation by using two T/C in both leads. In order to obtain a higher output emf, two or more thermocouples can be connected in series as shown in figure and for measurement of average temperature, a parallel connection as shown in figure may be used. Whenever compensation for variation of cold junction temperature is required, a wheatstone bridge circuit as shown in figure is used with one of its arms having a resistor of nickel wire or a thermistor and located near the cold junction. For accuracy, it is better to calibrate each thermocouple and then use it for measurement of temperature. The temp./e.m.f. Characteristics of thermocouples are also shown below.
Semi conductor temperature sensor is also used for temperature measurement and it works on temperature sensitivity of semiconductors (i.e. geranium or silicon). Silicon has +ve temperature co-efficient (0.7% per OC) and possesses linearity of ± 0.5% in its operating range of –65OC – 200OC. Similarly germanium crystal doped with Arsenic, Gallium or Antimony are used for low and cryogenic temperatures in the range of 1K-35K. Thermistors are semiconductor compound temperature elements and are thermally sensitive as resistors, relatively small in size, having low thermal capacity & high speed of response and possess large resistance values. Thermisters depict resistivity of 10-1 to 109 Ω-cm. Depending on their composition; the thermisters have limited KORBA SIMULATOR
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application in the range 100oC to +300oC. Thermistors are used often for compensating electrical circuit to ambient temperature changes. Thermistors possess high value of temperature co-efficient compared to wire wound resistors and are mostly used in integrated circuits for obtaining change in resistances at desired ranges. PNEUMATIC INSTRUMENTATION Pneumatic instruments find its application in the preliminary system of measurements & control in process industries since inception of industrialization. Pneumatic systems have many features and characteristics e.g. it is most versatile but simple, very cheap, robust, needs less maintenance support, fast in operation, high torque and thrust etc. The pneumatic instruments work on the principle of accurate conversion of mechanical movement to a proportional pneumatic signal by use of flapper nozzle mechanism working on air pressure. The flapper nozzle works on force- balance principle and it uses negative feed back which is used to oppose the force of the measuring element and the feedback bellows expand till the force is balanced and there is no further increase in output pressure. Air can exhaust faster than passing through a restrictor, resulting in increased gauge pressure reading. The orifice (nozzle) is around 3 times larger in size to that of orifice (restrictor). When the flapper is positioned such as to seal off the nozzle, the pressure builds up equal to the supply air pressure. The actual movement of the flapper is very small about 0.02 mm only. Small change in supply pressure does not affect the output but the flapper movement is so small that even the slightest amount of wear on pivots or linkages affects the system performance. Electro-pneumatic converters (E/P) convert electrical signal in pneumatic form for operating/driving valves, dampers or the power cylinders. The high processing speed of electrical and electronics signals is contained in UCB/ACS panels while the fastness of pneumatic drive operation is interfaced by use of E/P converters. The E/P converters receive electrical signals and control the pneumatic air operated power cylinder, valve or systems. The Fielden E/P converter is a force balance device without feedback; setting up of the nozzle is critical in this E/P converter because of the lack of feedback. The device is supplied with air at 1.5 bar pressure and has restrictor and nozzle size ratios similar to a conventional flapper nozzle system i.e. 3:1.The beam is pivoted at one end whilst the other end is attached to a permanent magnet, the plug of the primary valve is also connected to the beam. The spring tension and the primary valve plug relative to the nozzle seat is adjusted for its’ Zero adjustment. E/P converters use low-level electric current (4-20 mA) signal for conversion to pneumatic air pressure in 3-15 p.s.i. or 0.2 –1.0 kg/cm2 range; conversion is done by the flapper-nozzle process as explained earlier. From the diagram, it is clear that current flows through the coil to produce a force that pulls the flapper down and close–off the gap. Adjustments of the springs and perhaps the position relative to the pivot to which they are attached allows the unit to be calibrated in range i.e. 4 mA for 0.2 kg/cm2 & 20 mA for 1.0 kg/cm2. Current is applied to the coil and a magnetic field is set up, (the strength of which depends upon the value of the current) the permanent magnet is forced down which brings the primary valve closer to its seat, KORBA SIMULATOR
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pressure builds up and forces the diaphragm down which seals off the exhaust valve and opens the secondary valve, resulting in an increase in output pressure. Excess pressure is vented through the exhaust, resulting in a drop in pressure. The magnet is provided with Oil damping for smooth operation.
PNEUMATIC INSTRUMENTS
TARGET TYPE TXR WITH AIR RELAY
Air relays (Booster relays): Booster Air relays are used to supply large volume of air in the connecting pipe work and for the improvement in system response In Continuous bleed type air relays, the air continuously escapes via the vent. The rate of leakage determines the backpressure & the output pressure increases as the nozzle pressure increases in direct acting type and vice-versa in reverse acting type. i.e p out increases as the p nozzle decreases. In Non-bleed type air relays, the nozzle pressure is applied to the exterior of the large outer bellows and the control line pressure is exerted on the interior of the small bellows when the forces between the two are equal; the balance condition exists. This type of relay is used to provide a positive closing force. Any increase in primary nozzle pressure overcomes the spring pressure and tend to reduce the response time of relay. KORBA SIMULATOR
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Three Port Air relay consists of a two-lobe spool running in a surface ground cylinder, compressed air can be switched to the outlet by the application of a force to the spool. The force can be removed and the outlet will remain connected to the air supply. The application of a second force will return the spool to its original condition, main air will then be isolated and the air to the outlet will exhaust from third port. Five Port Relays is same as that of three port relays in its’ basic construction, the only difference being the use of a three- lobe spool. Compressed air can now be routed through the relay whilst at the same time a signal can be exhausted through it. The direction of force determines the routing of supply and exhaust.
Continuous Bleed Type Relay
Continuous Non-Bleed Type Relay
PNEUMATIC POWER CYLINDERS Power cylinders are single acting or double acting as per their functions. Single Acting Cylinder is the simplest type of power cylinder. In this type, air is used to make the piston unit out stroke or extend (+). Once the pressure has been removed, the return or in stroke (-) is achieved by mechanical means, in this case a spring. The cylinder can be air to extend type (application of a signal will push the piston out) or air to retract type (signal application pushes the piston in). Electropneumatic Solenoid converts electrical signal into mechanical rectilinear motion i.e. in a straight line. The solenoid consists of a coil and plunger. The plunger is free standing and is spring loaded. The coil operates on AC/DC specified supply voltage/current. The force of full or push is important parameter in deciding solenoid specifications. Solenoids are of continuous or intermittent rating and of single coil or double coils with specific duty cycle type i.e. percentage on total time is specified for particular type of application. SCR are normally used to activate the solenoid coil. Normally, the Solenoid operated power cylinders are single acting or air to open or air to close type and are termed as Shut off cylinders. Double Acting Cylinder is different than single acting in operation point of view that the power cylinder requires air pressure to move in both directions. In the double acting cylinder, if air is applied to Port-1 (with Port-2 open to exhaust) the piston will outstroke (+); and if air is applied to port-2 (with port-1 open to exhaust), the piston KORBA SIMULATOR
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will in stroke (-); (+)&(-) i.e up & down indicate cylinder movement. All regulating power cylinders are generally double acting. The Power Cylinder out strokes under the action of applied pressure, air is displaced from the other side of the piston to atmosphere through the main port and needle valve. When the cushioning boss enters the cushioning seal, the main port is blocked off, air can, therefore, only escape through the needle valve at a much slower rate, thereby, causing the piston to slow down for the premium period of travel. This results in the cushioning effects on high-pressure systems. Piston speeds can be in the order of 450 mm/sec applied to Port-2 (with Port-1 open to exhaust) the piston travels up. Impact forces at the ends of the stroke can be great. In order that damage may not be caused by sudden contact between the fast moving piston and the cylinder end housing, some form of buffer or cushioning can be used. This does not limit the piston travel but allows gradual deceleration in the last 25 mm or so of travel. In Kent Mark IV Pneumatic Power Cylinder, the Electronic controller output acts onto the E/P converter that converts the signal current (4-20 mA) into proportional air (0.2-1.0 Kg/cm2) pressure and is passed on to input bellows; it effects bellows to expand. As, the pilot valve spool is attached to the bellows via a connecting link; this unbalances the spool of the three port pilot valve, which is also supplied with driving air of supply pressure regulated as per the size and thrust from the power cylinder. Assuming an increase in the controller output, the Driving Air through the 3-port pilot valve is then admitted to the top of the piston and piston begins to move down. In doing so it takes the cam with it, as the cam moves down the bell crank lever turns about its pivot and through the spring opposes the movement of the bellows and restores the spool of the pilot valve to its original position. This way the system is the back in equilibrium. An equalizing valve is included to enable manual positioning of the piston. Wherever direct regulation by pneumatic system, is needed the signal air is directly sent to the bellows by a pneumatic regulator and the process repeats same as through E/P converter, explained earlier. Refer figures In Bailey Pneumatic Power Cylinder, the controller output acts on the bellows the spool assuming an increase in the controller output the bellows will expand and put the pilot beam up against the restraining force of the spring. This unbalances the pilot valve and causes air to be admitted to the top of the piston. The piston, therefore, begins to move down, this results in the positioner drive arm turn the cam, which puts more tension on the spring and so restores the pilot beam to its original position. Refer figure for details
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Kent Mark-4 Pneumatic power Cylinder
Bailey Pneumatic power Cylinder
Pneumatic (Valve) Positioners are fitted in pneumatic power cylinders and also in the pneumatic valves. These operate generally on a pilot valve principle. Since a pneumatic actuator requires a large power input to produce a large power output, it is necessary that sufficient quantity of pressurized air is made available to the large sized diaphragms or the power cylinder. Positioners are used for supporting the pilot valves. The pilot valve is attached to the pilot beam at one end whilst the other end is anchored and pivoted. Diaphragm Control Valves: A control valve must be capable of responding smoothly and rapidly to small changes in the controller output pressure signal. The quality of control will be impaired if any force, for example, that due to friction of working parts, oppose the movement of the spindle and the valve plug. The Diaphragm valves contain flexible diaphragms. The diaphragm virtually seals the chamber into two parts, the upper sections receiving the pneumatic signals from the controller via the air input. The input signals deflect the diaphragm, which is fixed to the thrust plate. The spindle attached to the thrust plate extends downwards into the body of the valve. There are three basic types of the valves: Quick Opening type diaphragm valves are used predominantly for full ON or full OFF control applications. A relatively small motion of valve stem results in maximum possible flow-rate through the valve; 90% of flow rate with only a 30% stem travel. Linear Type diaphragm valves have flow rate that varies linearly with the stem position. The flow relation is Q/Qmax = S/Smax; flow rate versus stem positions. Equal Percentage type of valves has equal percentage change in stem position that produces an equivalent change in flow type characteristic. KORBA SIMULATOR
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The primary function of a Pneumatic Valve Positioner is to ensure that the control valve plug position is always directly proportional to the value of the controller output pressure, regardless of gland friction, actuator hysteresis, off-balance of forces on the valve plug etc. This is achieved by incorporating a feed back lever that acts in opposition to the movement to the input. Since the spindle is connected to valve plugs, there is provision of automatically adjusting the orifice area in response to action from the controller and thereby altering the block valves (Diaphragm type valves). The deflection is opposed by the range spring whose stiffness constant and rating determines the extent of travel of the spindle for a given pressure range and effective diaphragm area.
AIR RELAY & CONTROL VALVE SET-UP
PNEUMATIC DIAPHRAGM CONTROL VALVE
This output pressure continues to increase until the valve spindle moves; mechanical feedback then restores the equilibrium. This force applied to move the valve spindle is sufficient to overcome the effect of all forces, no matter what the origin, which tend to oppose the spindle movement. Without the positioner the slight change in controller output signal may have been too small to initiate any corrective action. The matching of input signal range to valve travel range is achieved by changing the ratio of bellows/nozzle distance to feedback arm/nozzle distance. The controller output signal does not directly actuate the valve stem but is fed to a bellows unit. Assume that the system is in equilibrium and then the controller output increases slightly. The flapper is moved towards the nozzle and the relay output pressure begins to increase.
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Pneumatic Controllers Pneumatic Proportional action control; The principle of nozzle-flapper applies with the proportional controllers also. The input (0.2- 1.0 kg/cm2 air) changes then the input bellows expand/contract and that in turn forces the flapper to move the nozzle appropriately and balances the forces on the flapper/nozzle, the controller output pressure (range same as above) changes in the nozzle piping. The standard response of the pneumatic controllers is the Gain Kp =(x1/x2)(A1/A2); where A1, A2= effective area of the input & feedback bellows; x1, x2= Level arm of input and feedback lever arm. The proportional gain Kp can be changed by changing x1; since the bellows are having fixed geometry, the gain Kp is similar to the gain factor of an operational amplifiers. Pneumatic Integral action control; is achieved by using a variable restrictor in feedback loop (termed as integral bellow) and fitted in opposition to the proportional bellow. Pneumatic Derivative action control; is achieved by replacing the integral bellow with a spring and a variable restrictor in between the proportional bellow and the nozzle loop. Pneumatic Proportional Plus Integral Plus Derivative Control; It is obtained by suitably adjusting the proportional gain chamber bellows, integral and derivative action control module restrictors. The response curve drawn below shows that the final output of PID controller reduces the recovery time to approximately half to that of proportional action and the damped oscillation is resulted nearly at the set point only. The proportional plus integral action control mode slows down the process and thus the recovery time is increased quite more although the offset due to proportional action is reduced fully.
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PNEUMATIC PROPORTIONAL + INTEGRAL+ DERIVATIVE CONTROLLER
Electrical Actuators or the Pneumatic Actuators translate the control signal in to the driving force for dampers/valves.. Pneumatic, Electrical and Hydraulic actuators are classified according to the medium of application. The Electrical actuators are either the solenoids operated or the electrical motor driven actuators for driving the valve / dampers of mechanical types. The Hydraulic actuators use pressurized oil to operate the valve and the principle of its working is similar to the pneumatic actuators, except hydraulic pump units for pressuring, blocking units and filters etc are used. Electrical Motors are used for driving the actuators by producing the continuous rotation of motors. The style and size of the motor are decided on the basis of starting torque, rotation torque, rotational speed etc. The normal AC induction motors or the stepper motors are used for actuator application. The rate of rotation is determined by the AC line frequency and the rotor continues in rotation & in phase with line frequency. General details of electrical motors can be referred from other electrical equipments
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Electrical Interferences (random or periodic) create problems for control and measurements. The power levels of interferences are extremely low but they become significant for elimination. There are two modes of interferences: i) Differential mode is in series of interference that causes one signal lead (wire) in potential relative to the second signal and ii) Common mode interference appears between both signal leads and ground and causes the potential of both sides of the signal transmission circuit to be changed simultaneously and by the same amount relative to ground. Interference is usually coupled by conduction through metallic, resistive and capacity links or by radiation via stray capacitance and mutual induction effects. The conductance-coupling path is always stray. Grounding of the transducer is achieved by mounting its case and through the mains power lead. The ground potential differences cause common mode interference current which reaches the signal circuit through the conductor. Ground connection comprise the most common and most troublesome sources in measuring system which can be eliminated by providing single point grounding preferably at the source end. Electrostatic coupling is resulted due to capacitance between two objects and so such interference is associated with capacity leakage paths to ground. Reduction of circuit impedance (of signal) is the effective way of controlling the interference. Screening & shielding is second very important method for controlling capacitive coupled common mode interference. Screens or guard shields are situated and connected in such a way that interference current are returned to their sources without entering the guarded signal path. Coaxial, double coaxial lead and the twin conductor with screens are used for screening. At high frequencies care should be taken to match the impedance of the cable to that of connectors to avoid reflection and distortion of the signal. Signal conditioning circuits such as amplifiers may require external power, but be screened from the power supply. Common mode rejection can be used to reject the common voltage and provide an amplified signal for a grounded indicator. A conductor carrying AC is surrounded by an alternating magnetic field and voltages are induced in any conducting material position within the magnetic field. They require magnetic shielding to provide high permeability path for diverting any interference of the magnetic flux. Electro-static screens provide the protection from electro-magnetic interference. Addition of interference signals is avoided. Where cable screens are used, earth continuity of screens must be maintained through the installations with the earthling at one point only i.e. in the control room. At the field end the cable screen should be cut back and tapped/floating from earth. Intrinsically earth system be earthed in its own earth bar, static earthling be connected to common plant earth.
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SHIELDING& SCREENING
INSTRUMENT EARTHING FOR COMPUTERS
Signal conditioning (Data Acquisition) and Signal processing Signal conditioning (Data Acquisition) and Signal processing are defined as the excitation and amplification system for the Passive transducers and the amplifiers for the Active transducers so as to bring up the level of transducer to a sufficient quantum for conversion, processing, indicating and recording. Since the current carrying lines are also the power delivery lines to energise the transducer so, laying only 2 wires to connect the transducers does local signal conditioning and signal conditioned measurement system to the rest of the loop. Following three factors are important for conditioning: The load impedance should remain between 0-1000 ohms The interchange-ability of the process controllers; as the controller sees a 4-20 milliamp input signal and The measurement of power supply If the input is a variable resistance and a bridge or divider is used, the effect of nonlinearity with resistance in output voltage and the effect of current through the resistive sensor are considered. Any possible loading of voltage sources by the signal conditioning be always considered, since such loading is a direct error for the system. The potentiometers, strain gauges, resistance thermometers, inductive and capacitive transducers require excitation whereas thermocouples, techo-generators, inductive pickups, piezoelectric crystals produce their own voltage which only requires amplification and therefore are termed as active transducers.
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The DC amplifiers need differential inputs in CMRR mode. They are easy to be calibrated at low frequencies and have ability to recover rapidly from an overload condition. Carrier type AC conditioning systems for use with variable resistance or variable inductance transducers require carrier frequencies. Active filters can be used to reject the frequency and prevent overloading of the AC amplifier. DC systems are generally used for common resistance transducers (strain gauges) while AC systems are essential for the variable transducers and systems that have long lead lengths for transducers to the signal conditioning equipments. After signal conditioning physical quantities such as pressure, temperature, strain and positions are transformed into an electrical voltage or current of sufficient level for further processing in electronic circuits and indicators or the recorders. Almost all transducers are followed by signal-conditioning equipment. Resistance transducers are often placed in a D.C. Wheatstone bridge, while push-pull reactive transducers are placed in special a.c. bridges such as the Blumlein Bridge, producing a.c. carrier systems. The outputs of these bridges and signals from active transducers are usually amplified. Feedback around D.C. Operational amplifiers are used to obtain well-defined gain, and balanced differential inputs give high rejection to common-mode signals. Special amplifiers can provide particular requirements such as very low drift, isolation, and operation with high–impedance transducers. After amplification signal may be sampled, quantized, multiplexed and converted to digital form for storage, computation or transmission. Special care is needed to avoid addition of interference to the signal, and special equipment grounding, electrostatic screening and electromagnetic shielding may be necessary. Signal Conditioning – Design guidelines The guidelines as given below can be important in Signal Conditioning and designing. Thus enough information is made available to address an issue properly and exercise good technical judgment. Mostly, the entire system is developed, from selecting the sensor to designing the signal conditioning. Since the sensor is selected from what is available, the actual design is really for the signal conditioning. Guidelines for analog signal conditioning design: 1.
Define the measurement objective. a) Parameter: What is the nature of the measured variable: pressure, temperature, flow, level, voltage, current, resistance etc.? b) Range: What is the range of the measurement: 100 to 200OC, 45 to 85 psi, 2 to 4 V, etc.? c) Accuracy: What is the required accuracy: 5% FS, 3% of reading, etc.? d) Linearity: Must the measurement output be linear? e) Noise: What is the noise level and frequency spectrum of the measurement environment?
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2.
Select a sensor (if applicable) a) Parameter: What is the nature of the sensor output: resistance, voltage, etc.? b) Transfer function: What is the relationship between the sensor output and the measured variable: linear, graphical, equation, accuracy, etc.? c) Time response: What is the time response of the sensor: first-order time constant, second-order damping, and frequency? d) Range: What is the range of sensor parameter output for the given measurement range? e) Power: What is the Power specification of the sensor: resistive dissipation maximum, current draw, etc.?
3.
Design the analogue signal conditioning (S/C) a) Parameter: What is the nature of the desired output? The most common is voltage, but current and frequency are sometimes specified. In the latter cases, conversion to voltage is still often a first step. b) Range: What is the desired range of the output parameter (e.g. 0 to 5 V, 4 to 20 mA, 5 to 10 kHz)? c) Input impedance: What input impedance should the S/c present to the input signal source? This is very important in preventing loading of a voltage input. d) Output impedance: As offered at short-circuiting on loaded circuit?
OPERATIONAL AMPLIFIERS (OP-AMP) An Operational amplifier is the name given to the electronic amplifier that is employed to analogue computers for performing mathematical operations. Sophisticated Integrated Circuit (IC) chips fabricated on single chip of semiconductors ( Si, Ge ) are the basic elements of the Op-Amp. An Op-Amp is direct coupled, high gain voltage amplifier, designed to amplify the signals over a large/ wide frequency range. It has two input terminals and one output terminal and a gain of at least 10 5. The ideal Op-Amp is a linear device in that the output is directly proportional to the inputs for all values of input voltage. Practically the relationship v0 =- Kvi holds true where K =zo/zi; An Op-Amp is basically the differential amplifier which responds to the difference in the voltages applied to the positive and negative input terminals. Basic Op-amp application and the circuits are given in paragraphs that follow: In the Non- Inverting configuration, an Op-amplifier is connected in single ended input and the input impedance is high by virtue of the feedback, the output signal appears in same sign of the input signal. We use the resistors as the input & feedback impedances as shown in the fig. In Laplace representation form VI (s) = V1(s) - (Rf/R1) V1(s) and the output is given by ( vi ) = v1 - (R1/ R1+ Rf) vo In the Inverting mode which uses an Op-amplifier as a sign inverter, thus the output signal appears in sign inverted of the input signal; resistors are used in the input & feedback impedances as shown. In Laplace representation form Vo (s) = (Rf/R1)V1(s) and the output is given by ( vo ) = - (Rf/R1). v1.
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Basic Op-amp.
Non-inverting Op-amp.
Inverting Op-amp.
In the Voltage follower configuration, the output voltage follows the input. This is basically the more refined version of emitter follower or the source follower; its gain becomes 1, and so the Circuit gain Av = (vo)/(vi) =1. Since the feedback resistor Rf =0 and input resistor R1 is α (open circuit). It also performs the action of an Impedance Transformer. In the current to voltage converters, the current [Is] is introduced into the (–) ve terminal of the Op-amp, an equal current in the feed back resistor is setup and since the amplifier input voltage (v1) is practically zero, thus the source current has been converted into a voltage, and the output voltage is ( vo ) = -Is.Rf In the voltage to current converters, the voltage source of high internal resistance drives a low-resistance load; no current flows into the op-amp terminals & no voltage drop across the resistor thus full voltage appears between the + terminal and ground; the source voltage gets converted into a load current. The Load current IL and voltage source Vs is related: IL= Vs/R1
Voltage follower
Current to voltage convertor.
Voltage to current convr.
In the Integrator, the input signal (v1) is integrated to obtain the output signal (vo). In integrating, the initial conditions are added and are initially biased by a dc voltage. A capacitor is used as the feedback impedance and a resistor is used as the input impedance In Laplace representation form Vo (s) = - V1(s). (1/RiCS) and the output vo =1/ (R1 C). v1 dt In the Differentiator, the input signal ( v1 ) gets differentiated to obtain the output signal (vo), the resistance and the capacitors in the circuit are inter-changed to form the differentiator as shown in the figure; in Laplace representation form Vo(s)=Vi(s).RiCS and the output vo = - R C1. (d v1/ dt)
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In the Summing Op-amp, several inputs can be summed up by using resistors in the input as given in Inverting configuration and feedback impedances of the high gain d.c op-amp. The output voltage (vo) of the multi-input voltages is given by (vo) = K {(v1) + (v2) + (vn)} = where K=Ro/Rz
Integrator Op-amp
Differentiator Op-amp
Summing Op-amp.
In the Waveform generators, Triangular wave, Square-wave, pulse train, etc. can be generated by the use of one Op-Amp and a pair of back to back Zenor diodes and some filter elements, input may be a simple sinusoidal signal. While designing the op-amps for typical waveform generation, an appropriate type as per required application of op-amp along with the power supply and the feedback network is selected.
Comparator & Clipper Op- Amp.
Waveform Generator Op- Amp.
Square wave Generator Op- Amp.
Triangular-wave Generator Op- Amp.
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TURBINE SUPERVISORY EQUIPMENTS The advantage of equipping turbines with supervisory instrumentation was recognised some forty or more years ago and at that time it was envisaged that such instrumentation would provide accurate and easily interpreted indications of the mechanical behaviour and working clearances of steam turbine generating plant. However, in the context, of turbine start up the records obtained are peculiar to the units concerned and are of particular value in providing a record of past operation on which to base future procedures. The supervisory equipment supplied with large steam turbines involves the application of various machine-mounted detectors to provide the necessary information on temperature differentials, running clearances and dynamic balance of the turbine/shaft and to ensure safe start up, operation and shut down within minimum time. The detectors of measurement of parameters mounted outside of the steam space e.g. shaft eccentricity; axial differential expansion and vibration all give little or no information on the situation inside of the cylinders. Nevertheless, turbine supervisory equipment are regarded as essential in providing a warning of imminent trouble, as well as an indication and permanent record of the behaviour of turbine generating plant during start up and running on line; the thermal stresses, expansions and vibrations should not become excessive such that turbines are safe and defect free. Under changing conditions, the rate of heating or cooling of the rotating and stationary parts of steam turbines is not the same and differential expansions and contractions occur, which for example, tend to close the small axial clearances between the rotating and stationary blades. Furthermore, surfaces exposed to the live steam cause change in temperature more rapidly than the rest of the turbine and if temperature differences are not kept within satisfactory limits, plastic strains with cracking of the affected parts may result in the long term. Reliability and repeatability are of overriding importance in establishing the confidence of operatives in turbine supervisory equipment. A few of the turbovisory instruments have been described in the paragraphs that follows: Differential Expansion Measurement: Historically, the measurement of axial differential expansion between turbine rotors and cylinders has been significant as an operating parameter. With the increased length of machines, however, the methods of measurement based on the use of both double taper and straight radial collars on the shaft have sometimes presented major problems in the realisation of acceptable accuracy. To deal with this situation a system-employing eddy current probe has been developed which does not suffer from drift. Such detectors are mounted on the bearing keep so as to present the probes appropriately to the periphery of the specially machined collar on the turbine shaft. Changes of shaft behaviour /attitude and eccentricity have no effect on the accuracy of measurement so long as the probe gap remains within its working range. Vibration Detection and Measurement: Pedestal vibration analysis can give information of the shaft condition (whether cracked/may develop crack within some period). The vibration is measured in Amplitude, frequency & Phase. In a vibration KORBA SIMULATOR
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detector the voltage is induced across the end of a conductor that moves through a magnetic field. The induced voltage is proportional to the relative velocity between the coil and magnetic field and it depends on the coil length, magnetic strength and velocity of passing magnetic field. Permanent magnet is firmly attached to the case; it follows the vibratory motion. Coil assembly is supported by a spring suspension system and combined mass is designed to have low natural frequency. Induced voltage is proportional to vibration velocity (measured so) and displacement can be obtained by electronically integrating the signal. Shaft vibration is defined as the dynamic movement of the shaft around the eccentricity locus. And by this measurement routine operational monitoring of changes in vibrational behaviour and radial clearances of the machine can be ascertained.
Differential Expansion Detector
Vibration Measurement Detector
Eccentricity Detector
Measurement
The Vibration measurement can be made in three basic quantities as under; 1. Amplitude: It refers to the level of vibration; it can be further represented by i)
Displacement which is the total distance the vibrating part moves in a given direction, its unit of measurement is micrometer (m-6)
ii)
Velocity refers to the speed of the part at which it moves at any instant during the vibration cycle ( 0 to max or peak); measured in r.m.s & its unit is mm /sec Acceleration refers to the rate of change of vibration velocity at any instant during the vibration cycle; it is measured in r.m.s and its unit is mm/sec/sec
iii)
2. Frequency: Vibration in rotating machines occurs as a result of imbalance in the forces generated or acted upon in the M/C and the frequency is in multiples of shaft rotational speed which ranges from 10 Hz (close to 1st critical speed of 500MW T.G sets) to 750 Hz (new revised figures reached are 5-1000 Hz) 3. Phase: It refers to the angle of unbalance and is given in radian/degree. Normally the Piezo-electric (earlier explained) type sensors are used as vibration pick-up .As explained earlier, small variation in construction of vibration pick-up i.e. the spring stiffness and in magnet characteristics etc gives rise to output voltage
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variations. Magnetic effects are picked up in considerable quantum due to the a.c. generator vicinity and may create noise, but this can be overcome by incorporating magnetic screen with the instrument. Depending upon the overall sensitivity, vibration instrument may give an output reading when subjected to high cross axis excitation. On a turbine pedestal for example, the Horizontal and Vertical vibration may be very similar in magnitude, in this circumstances the transducer with high transverse (cross-axis) sensitivity could produce an output which is significantly different from the actual vibration level in the required direction.(Cross-axis figure should not exceed 10 %) While going for Vibration Measurement, common practice is followed to measure bearing pedestal vibration which, though dependent to some degree on the mass of the pedestal and the rigidity of the foundations, is particularly sensitive to out of balance forces on the turbine shaft at the higher speeds. Vibration amplitude for a certain level of severity must get smaller and smaller and the alarm level changed accordingly. It provides signals directly proportional to acceleration and offers particular advantages of robustness and resistance to environmental conditions. The responses to imbalance, thermal bends, rubs & unloading of bearings are more clearly observed by measurements of shaft movements within the bearings rather than by the vibration of pedestal in the steam turbines. It is measured as the diameter of the locus traced by the shaft centre; the change in radial air gap within the cylinder is inferred from eccentricity. Shaft eccentricity equipment indicates the amplitude of shaft vibrations relative to the pedestal, in one direction, at several points along the rotor, all of them outside the steam space and close to a bearing. In the limit, rubbing may occur with the resultant localised heating possibly leading to serious damage. The detection of shaft eccentricity is particularly valuable at the lower speeds in establishing the straightness of a turbine shaft but the measurement is often complicated by phenomena of shaft whirl, which can arise from effects originating at the bearings. For shaft eccentricity measurement an eddy-current probe is presented to the periphery of the shaft at a defined longitudinal distance from the centre line of the bearing. As permanent, continuously operating monitoring equipment, this is extremely useful, but for indicating the onset of shaft vibration trouble on a particular turbine it is not so reliable. At all times, the shafts of steam turbines must conform within close limits to the natural deflections due to their own weights. If the temperature distribution in a shaft is not symmetrical about it’s axis differential expansion will cause the shaft to bend and if the speed is raised centrifugal forces will aggravate the condition. Normally the Non-contacting type proximity sensors are used which contains transducer, connecting cable and electronic processing units; these transducers operate on eddy current or the variable reluctance principle. LVDT has been explained Thrust Position (Shift) Measurement: The primary purpose of thrust position measurement of the turbine shaft is to provide an accurate indication of the onset of thrust bearing failure. With such an indication giving advanced warning the damage KORBA SIMULATOR
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to the machine can be limited to only the loss of the thrust bearing. The measurement is made, as near to the thrust bearing as possible since the greater the distance between the thrust collar and the points of measurement, the greater will be the inaccuracy. This is brought about by thermal growth and various pressure strains on the shaft and the turbine casing. If possible the thrust probe should observe the thrust collar directly. The probe should be installable, removable, and its gap adjustable from the outside of the bearing housing with the machine in operation however, this is not always possible. Shaft Eccentricity Measurement: Eccentricity is defined as the out of centre Turbine Rotor &Casing Expansion Measurement: The front bearing housing of the HP and IP turbines can slide on their base plates in an axial direction. Fitted keys prevent any lateral movement perpendicular to the machine axis. The bearing housings are connected to the HP and IP turbine casings by guides which ensure that the turbine casings remain in their central position while at the same time axial movement takes place. Thus the origin of the cumulative expansion of the casings is at the front bearing housing of the LP turbine. The casings of the LP turbine are separately and axially located by fitted keys at the front supports of their longitudinal beam members on the base plates. Free lateral expansion is allowed. The centre guides for these casings are recessed in the foundation crossbeams. There is no restriction on axial movement of the casings. At the rear supports of the longitudinal beam members the casing is free to expand horizontally in any direction. Hence, when there is a temperature rise, the outer casing of the LP turbine expands from its fixed points towards the generator. Differences in expansion between the outer casing and the fixed bearing housings to which the housings for the shift glands are attached are taken by bellows type expansion joints. The IP and HP casings expand towards the front bearing housing of the HP turbine as the fix point of the turbine casing on the foundation is at the bearing housing between the IP and LP turbines. Also the fixed point of L.P Turbine is there at the bearing casing housing front base plate support of each longitudinal beam member of the LP turbine; from these points the members and the LP outer casing bolted to them expand towards the generator. The thrust bearing is incorporated in the front bearing housing of the IP turbine. Since this bearing housing is free to slide on the base plate the shafting system moves with it. Seen from this point, both the rotor and casing of the HP turbine expand towards the front bearing of the HP turbine. The rotor and casing of the LP turbine expand towards the generator in a similar manner. The movement and expansion of individual casings is measured to ensure that the alignment between pedestals is maintained within acceptable limits. In turbines when the HP and IP casings heat up, they expand in the direction towards HP end, so each LP casing expands towards the generator end. Rotor expansion takes place in the direction away from the location point within the thrust block assembly. The rotors can, either expand within the clearances provided at the pedestals or can be
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keyed to the casing to move axially with the expanding rotor depending on the turbine designs. Rotor Temperatures: The HP and IP rotors may be subjected to high thermal stresses due to temperature gradients through the rotor assemblies. These conditions arise during start-up when high temperature steam flows over relatively cool rotors. Several methods are adopted to predict the temperature gradient during turbine run-up. One way is to rely on heat transfer equations written into the computer software, other way is to make use of measurements taken in the HP and IP steam pipes located in positions within the pipe work in such a way that their response over short periods is the same as that of the rotor. Metal temperatures are displayed as individual temperatures or differential temperatures from combinations of individual or average temperatures. CHEMICAL MEASURING INSTRUMENTS Chemical control of water systems plays an important part in the operation of modern power plant. Without it plant would deteriorate rapidly and major failures would soon result leading to loss of availability. The basic aim of chemical control is to maintain an environment in all parts of the system, and the material used for all conditions of operation. This is achieved by minimizing the impurity ingress, removing impurities & corrosion products etc. thereby maintaining high water purity & Chemical conditioning. Chemical control of the feed water system is achieved by addition of ammonia and hydrazine concentrations in the feed water. pH measurements provide indication of adequacy of ammonia dosing & necessity of continuous hydrazine monitoring. The electro chemical technique is adopted for measuring the concentration of a component in solution in terms of effect it has on the electrical properties of the cell. The effect therefore forms the basis of the electrochemical transduction of the active type and the e.m.f. developed between the two electrodes of the cell constituted for this purpose is used to signify the concentration level. A galvanic cell is used as a converter of chemical energy into electrical energy, whereas an electrolytic cell is supplied with electrical energy from an external source. At the exact point at which the galvanic e.m.f is balanced by the applied e.m.f., no current flows through the cell, and under these conditions, the potential of each electrode reflects the composition of the solution. Chemical monitoring instrumentation monitors the chemical conditions of the system during operation and facilitates chemical control of the water treatment plant, the condensate polishing plant etc. and alarms the operator of any marked departure from acceptable conditions and provides information for troubleshooting. Some of important application of chemical instrumentations at various plant locations are shown and described in short in pages below.
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The Make up Water Treatment Plant. Conductivity (before & after cat-ion resin) reactive silica and sodium measurements are done for finding out the performance of individual items of plant. It gives warning of resin bed exhaustion and ensures required water quality. In addition pH equipment is often installed at sumps etc. to ensure the neutrality of acid and alkali spent by re-generants prior to disposal. The Condenser: Direct conductivity to facilitate in finding the condenser leak locations by means of probes installed at various locations within the shell. The Condenser Extraction Pump: Direct and after cat-ion conductivity (supplemented by sodium in some instances) to provide warning of condenser leakage and the Dissolved oxygen to ensure adequacy of oxygen removal at the condenser. The Condensate Polishing Plant: Conductivity (before and after cat-ion resin) and reactive silica to facilitate plant operation and check on outlet water quality in addition to this chloride and sodium are monitored. Downstream of chemical dosing points: PH as a check on adequacy of dosing. Deaerator Inlet / Outlet: Occasional need for on-line monitoring of dissolved oxygen to check on deaerator performance. Final Feed water line: Conductivity, dissolved oxygen, pH, sodium and reactive silica as a check on the final feed water quality and its acceptability for feeding to the boiler. During commissioning and subsequent start-ups it may be considered beneficial to continuously measure the total iron (i.e. corrosion product) inventory to the boiler. In addition continuously monitoring of ammonia and hydrazine concentrations in the feed water since chemical control of the feed system is achieved by addition of these chemicals. Boiler Water (Drum type system): Conductivity (before and after cat-ion resin) chloride and sodium to ensure correct alkaline conditions in the bulk are being maintained. pH measurements provide an indication of the adequacy of ammonia KORBA SIMULATOR
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dosing and the necessity for continuous monitoring of hydrazine other than to economise on its use is (or has been in the past) debatable. This situation may change in the future as a result of excess hydrazine possibly being linked with erosion / corrosion problems currently being investigated. Main Steam: Conductivity (before and after cat-ion resin) and reactive silica to obtain a measure of salt carry over to the turbine blades. The latter may only be required during early operation to establish a relationship between silica in steam and boiler water once this relationship has been established then monitoring the silica in the boiler water may be adequate. Electro Chemical Transducers The concentration of a component in solution is measured in terms of effect it has on the electrical properties of the cell. The effect of the component on an electrode introduced into the solution forms the basis of the electrochemical transducer of the active type and the e.m.f developed between the two electrodes of the cell constituted for this purpose is used to signify the concentration level. Electrochemical transducers enable the presentation of ionic potentials into a suitable form so that output signals are representing the bio-chemical phenomena (in which both membrane barriers and metal electrolyte interfaces are used). The electrochemical cells are of two types. 1) Galvanic 2) Electrolytic. A galvanic cell is used as a converter of chemical energy into electrical energy, whereas an electrolytic cell is supplied with electrical energy from an external source. At the exact point at which the galvanic e.m.f is balanced by the applied e.m.f, no current flows through the cell, and under these conditions, the potential of each electrode reflects the composition of the solution. The theory of electrodes and the principles that govern their design are a little involved and require an understanding of electrochemistry. Measurement of ion concentration in electrolytes pose problems because an electrode is selective to desired ion species and its concentration in the solution and reference electrode remain independent of the variation of the electrolyte surrounding the indicator electrolyte and separating the electrolyte. The metal used for electrode is mercury, which is in contact with a paste containing equal weights of calomel and potassium chloride. A column of saturated KCL solution over the paste enables contact with the test solution through a salt bridge of KCL whose concentration is kept about 3.8 mol per litre. Contact with mercury pool is by means of a platinum wire which may be amalgamated Ag/AgCl or Calomel (Mercurous chloride) electrode is used for pH measurement of water. The porous plug or the salt bridge serves the function of connection between the reference and indicator electrode without allowing the solutions to mix with each other. However the passage of ion takes place through the unequal rates of diffusion develop some junction potential. Electrochemical transducers enable the presentation of ionic potentials into a suitable form so that output signals represent the bio-chemical phenomena (in which both membrane barriers and metal electrolyte interfaces are used) for pH KORBA SIMULATOR
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measurement of water the metal used for electrode is mercury, which is in contact with a paste containing equal weights of calomel and potassium chloride. A column of saturated KCL solution over the paste enables contact with the test solution through a salt bridge of Kcl whose concentration is kept about 3.8 mol per litres. Contact with mercury pool is by means of a platinum wire, which may be amalgamated Ag/AgCl or Calomel (Mercurous chloride) electrode. The porous plug or the salt bridge serves the function of connection between the reference and indicator electrode without allowing the solutions to mix with each other. However the passage of ion takes place through the unequal rates of diffusion develop some junction potential. Measurement of oxygen Although the concentration of oxygen in gases is the parameter of widest interest it should be remembered the most if not all, methods of measurement actually respond to oxygen partial pressure. Since instruments read directly in concentration units they must be calibrated at the pressure of measurement. Gas Absorption Methods These methods are dependant upon the oxygen in a measured volumetric sample of gas being absorbed by a chemical solution, normally alkaline pyrogallol solution. The resulting reduction in volume, measured at atmosphere pressure, indicates the volume of oxygen originally present and allows the concentration, (by volume) to be calculated. Apparatus for measuring oxygen by this method, along with other gases such as carbon monoxide and methane, are typically those referred to as the Orsat, Haldane, Bone and Wheeler and Schholander gas analysis apparatus. Of these the Orsat is the most common and although somewhat crude it still finds use in the laboratory and as a portable means of gas analysis on plant. The limitations of this type of apparatus are obvious and the accuracy of measurement is very dependent upon the skill of the operator. Paramagnetic Analysers Two of the electrons in the outer shell of the oxygen molecule are unpaired. The magnetic moment is therefore not neutralized and as a result, the oxygen molecule is strongly paramagnetic i.e. attracted by a magnetic field. (Most other gases are either unaffected by a magnetic field or exhibit weak diamagnetic properties i.e. they are repelled) There are two-three types of instrument for measuring oxygen, which make use of this property. Magnetic Wind Analyser utilizes the principle of paramagnetic property of O2 as hown in the diagram the sample gas enters at the bottom and passes upwards through the side tubes. Located in the cross tube is a heated filament which forms one arm of a Wheatstone bridge. Oxygen present in the sample gas is attracted from the left hand tube into the cross-tube by the magnetic field. On entering the cross-tube it is heated by the filament and this has the effect of reducing its paramagnetic susceptibility. The heated gas is pushed along the cross-tube by cold gas entering at the left thus creating a flow of gas along the cross tube proportional to the partial pressure of oxygen. The flow of gas cools the filament so changing its resistance. This change is KORBA SIMULATOR
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resistance unbalances the Wheatstone bridge giving a signal, which can be related to the oxygen concentration. Quincke Type Analyser: In this analyzer a continuous steam of nitrogen gas passes over two sections of a Wheatstone bridge. Flows are adjusted so that the heat losses over the two sections are the same and the Wheatstone bridge is balanced. The right hand stream of nitrogen passes through a magnetic field whilst the left hand stream passes through a similar volume, but without any magnetic field, into a common outlet. Sample gas is introduced upstream of this outlet and mixes with the nitrogen as it emerges from the two arms. The presence of oxygen in the sample gas generates a small backpressure on the right hand flow pressure distorts the flow pattern of nitrogen around the arms of the Wheatstone bridge which becomes unbalanced and gives a signal proportional to this oxygen content. It requires a continuous flow of nitrogen and accurate alignment to ensure that there is no gravitational chimney effect. Magneto-Dynamic Analysers also make use of the paramagnetic properties of oxygen but in a different way. A diamagnetic body located in a non-uniform magnetic field will be repelled by that field, the forces acting upon it being dependant upon the strength of the pole pieces and the magnetic susceptibility of the gas which surrounds it. The presence of a paramagnetic gas such as oxygen increases that susceptibility (and therefore the forces acting upon the diamagnetic body) in proportion to the oxygen partial pressure. This is the basis of the Magneto-Dynamic Analyser. The first measuring cell to take advantage of this effect was that developed by Pauling and known as the Pauling Dumb-bell system. The Pauling cell consists of two diamagnetic spheres of glass filled with nitrogen and mounted at the ends of a bar to form a dumb-bell. This dumb-bell is mounted horizontally on a vertical quartz fibre torsion suspension.
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The whole measuring cell operates inside a strong non-uniform magnetic field. The spheres are repelled from the strongest part of the field and so rotate the suspension until the force produced by the twist of the suspension is equal to the force acting on the spheres. A change in the oxygen content of the gas inside the cell changes the force acting on the dumb-bell causing it to take up a new position. This can be measured wither directly e.g. using a lamp/mirror system or electromagnetic feedback can be used to restore the dumb-bell to the zero position as in the Munday cell. The current required to maintain the dumb-bell in the zero position is a measure of the magnetic susceptibility of the gas present in the cell and therefore the oxygen partial pressure. This cell is the basis of the Servomex range of oxygen analysers. With magneto-dynamic analysers only the magnetic susceptibility of the gas is being measured and the reading is unaffected by changes in the carrier gas composition. The response is linear with oxygen partial pressure and two-point calibration usually with air and nitrogen (to set the zero of the instrument) is sufficient. Calibration and measurement at the same pressure enables a direct readout in concentration units to be obtained. The High Temperature Ceramic Sensor The High Temperature Ceramic Sensor is in principle an oxygen concentration cell comprising two electrically conducting; chemically inert electrodes (usually platinum) attached to either side of a solid electrolyte closed ended tube. This is shown schematically. The tube is completely gas tight and made of a ceramic (usually calcium stabilized zirconium oxide), which, at the temperature of operation, conducts electricity by means of oxygen ions. The potential difference across the cell is given by the Nerst equation i.e. E = 2.303 RT log( P1/ P2) /RTvolts , P1 & P2 oxygen partial pressure at electrodes F P2,, E is the potential difference; R is the gas constant (1.987 cal /deg /mole) T is the absolute temperature. F is Faradays constant (23060 cal/v) Thus, provided that the oxygen partial pressure is known at one electrode, then the potential difference between the two electrodes enables the unknown oxygen partial pressure to be determined at the other electrode. In practice air is invariably used as a reference gas and is allowed, or directed to come into contact with the one electrode whilst the other electrode is exposed to the sample gas. The high temperature ceramic sensor holds over a very wide range of oxygen partial pressure allowing measurements to be made in the percent oxygen range on the one hand through ppm concentrations to oxygen range partial pressure as low as 10-25 atm on the other. Oxygen reacts with combustible gases in the sample at above 600OC. The platinum outer electrode of the sensor acts as a catalyst in this respect and the output will therefore be a measure of the residual oxygen. The ceramics used for sensing leads to situation of even operating at a lower temperature.
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The measurement of oxygen in flue gas is an important aid to the combustion control of oil and coal-fired plant. The immediate problem that has to be overcome in this application, however, is the wet dirty and potentially corrosive nature of the gas to be analysed. The direct oxygen probe based on the high temperature ceramic sensor and manufactured by Kent and Westinghouse has the advantage that it can be installed directly into the flue gas ducting. Not only does this avoid the complication of a gas sampling system but high temperature operation ensures freedom from “dewing out” of water on the cell surfaces. Filtration of particulate matter can be more readily achieved and a further advantage is that oxygen reading on a “wet” basis can be obtained directly. A fast instrument response is also a feature of the Kent probe system as shown. For direct oxygen read out on a “wet” basis sampling system and measuring cell must be maintained above the dew point. A steam ejection sampling system of the type as shown is also employed to provide a sample suitable for oxygen measurements using a paramagnetic analyser
CONTROL SYSTEMS In any industrial operation, controlling the process parameters like pressure, flow, Level temperature, humidity, viscosity etc., and automatic control becomes essential; it has become the important and the integral part of the power plant (any process). Automatic controls provide means for attaining optimal performance of the dynamic system, improve the quality and economizes the cost, expand the production rate, reduces the repetitive manual operation. All above thus calls for good understanding of the field. As the efficient power- plant-operation requires handling many inputs and outputs, systems are becoming more complex and lot many equations are being incorporated to cope/ be in pace with the advancements in control field, the designing of control system must be such as to use complex computation through the electronic analog, digital and hybrid computers and on-line computers for
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operation support. Some terminologies appearing frequently are given below to describe the intricacy of control systems: Plant: A set of machine parts functioning together to perform a particular operation is considered as a Plant. Process: Any system comprised of changing and dynamic variables usually involved in manufacturing and production operation describes a Process. Systems: A combination of components that act together, but not limited to physical ones rather the abstract and the dynamic phenomena forms a System. The System implies to biological, economic, physical etc. Internal & External Disturbances: The signal that affect the output of the system, is termed as the Internal Disturbances whereas the External Disturbances refers to the disturbance generated outside the system although partly is an input only. Unpredictable disturbances are designated as unknown parameter beforehand; with predictable or known disturbances, it is always possible to include compensation within the system so that measurements are unnecessary. Servomechanism: is a feedback control system in which the output is some mechanical position, velocity, or acceleration etc.; in control system, the terms servomechanism and position are synonymous. Control Lag: The control system has Control Lag (it refers to the time) associated with its operation that must be compared to the process lag. Due to the control lag, when a controlled variable experiences a sudden change, the process-control loop reacts by outputting a command to the final control element to adopt a new value to compensate for the detected change. Control lag refers to the time for the processcontrol loop to make necessary adjustment to the final control element. Dead Time: Another time variable associated with process control that is both the function of the process-control system and the process is Dead Time. This is the elapsed time between the instant a deviation (error) occurs and the corrective action first occurs. The dead times can have a very profound effect on the performance of control operations on a process. Cycling: The dynamic behaviour of the variable error under various modes of control, leads to an oscillation of the error about zero. This means the variable is cycling above and below the set point value. If cycling continues indefinitely, this leads to the steady state cycling; in such situation we are interested in both the peak amplitude of the error and the period of the oscillation. We have cyclic transient error if the cycling amplitude decays to zero, We are interested in resolving and controlling the initial error, the period of cyclic oscillation, decay time for the error to reach etc. Feedback control: A feedback control in a control system is one which tends to maintain a prescribed relationship between the reference input( set point decided any where within the range) and the output by comparing these two signals and using the difference as an error input to the controller.
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Proportional Band (PB) is the change in proportional input that is required to produce full-scale change in output. E.g. The PB is 10 % if the 10% change in error causes 100 % change in output. Dynamic Response of control Devices is the dynamic behaviour, in response to a sudden change in load etc.; this is required to evaluate the range of regulation and tuning of the controller performance. Often, the changes are random and therefore unpredictable except on a statistical basis. One has to find complete response to a step function or alternatively the frequency response for steady-state sinusoidal inputs. Automatic regulating system employs feedback in which the reference input or the desired output is either constant or slowly varying with time Controllers are meant to maintain the actual output at the desired value in the presence of disturbances of automatic regulating system. Transducers are used to measure variables and convert them to electrical form. Computers or comparators are used in determining the difference between reference and feedback variables. Controllers and amplifiers are used to develop signals to actuate the controlled system. Motors are used to provide the ‘muscle’ for system control. Process control is essential requirement of any industry. Automatic control of pressure and of electric quantities such as voltage, current and frequency, programmed controls of temperature by heating furnaces according to a preset program etc. are often used in Process control systems. It should be noted that most Process control systems include servomechanisms as an integral part. Despite their great variety, control systems can be analysed into a few basically similar components. Working of the Watt’s Flywheel governor is the best example to understand the philosophy of the primitive process control system installed in the process industries. CONTROL SYSTEMS: MODES OF CONTROL ACTIONS The Control Systems work in two modes as explained below: Open Loop Control Mode: In this mode, the output has no effect upon the control action; the output is neither measured nor fed back for compensation with the input. For each reference input, there corresponds a fixed operating condition as the o/p is not compared with the input and the system accuracy depends on calibration. It operates on a time basis that is why it is termed as open loop system Closed loop control mode: is the mode in which the output signal has a direct effect upon the control action. System requires a feed back signal. Error signal between the input and the feedback signal is fed to the Controller so as to output to the system for reducing/neutralising the error and bringing the output of the system to a desired value. In a closed loop system use of feed back signal reduces the system error. In comparison to an open loop system, the closed loop is capable of greater accuracy over a wider range of conditions even when less precise control elements are used. In Watt’s flyball governor, it being perhaps the first automatic control device, the governor of the engine drives a rotating spindle carrying the flyweights. The engine KORBA SIMULATOR
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speeds up until the centrifugal force of the flyweights overcome the force of the speed adjusting spring and partially closes the throttle valve. An increase in load on the engine momentarily reduces its speed, reduces the centrifugal force of flyweights, allows the throttle valve to open, and accelerates the engine until the set speed is reached. Block diagram of a fly ball governor
Load Frequency Control: The figure below shows the single turbo-generator system supplying an isolated load. The system consists of components: Fly ball Speed governor system senses the change in speed (frequency); as the speed increases, the fly balls move outwards and the point B on linkage mechanism moves downwards and reverse happens in decreasing of speed. Hydraulic Amplifier has a pilot valve and main piston arrangements. Low power level pilot valve movement is converted into high power level piston valve movement. For opening /closing the steam valve against high-pressure steam. Linkage Mechanism: ABC is a rigid link pivoted at B and CDE is another rigid link pivoted at D. This link mechanism provides a movement to the control valve in proportion to change in speed. It also provides a feedback from the steam valve movement (link4). Speed changer: It provides a steady power output setting for the turbine. Its downward movement opens the upper pilot valve so that more steam is admitted to the turbine under steady conditions (hence more steady power output). The reverse happens for upward movement of speed changer. The point A on the linkage mechanism is moved downward by yA then yA = kC PC ; PC is commanded increase in power which sets into motion a sequence of events like pilot valve moving upward, high pressure oil flowing under top causing main piston moving downward, the steam valve opening causing increase in turbine, generator speed and frequency goes up. 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 high-pressure oil into the cylinder thus moving the main piston & opening the steam valve by yE.
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Certain justifiable simplifying assumptions, which can be made, are: Inertial reaction forces of main piston and steam valve are negligible compared to the forces exerted on the piston by high-pressure oil. Because of (i) above the rate of oil admitted to the cylinder is proportional to port opening yD. The volume of oil admitted to the cylinder is thus proportional to the time integral of yD. The movement of yE is obtained by dividing the oil volume by the area of the cross-section of the piston. Thus yE = k5 (- yD) dt. (i) From the schematic diagram (left side), it is observed that a positive movement yD creates a negative (upward) movement yE accounting for the negative sign in (i) above. Load frequency and excitation voltage control: This control is non-interactive for small changes and can be modelled and analysed independently. Furthermore, excitation voltage control is fast acting in which the major time constant encountered is that of the generator field; while the power frequency control is slow acting with major time constant contributed by the turbine and generator moment of inertia; this time constant is much larger than that of the generator field. Thus, the transients in excitation voltage control vanish much faster and do not affect the dynamics of power frequency control. Changes in load demand can be identified as: (i) slow varying changes in mean demand, and (ii) fast random variations around the mean. The regulators be designed to function insensitive to fast random changes, otherwise the system will be prone to hunting resulting in excessive wear and tear of rotating machines and control equipment.
Schematic Diagram of Load– Frequency Control
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CONTROL SYSTEMS: CONTROLLERS Controllers process data that are input from the sensor, apply the logic of control, and cause an output action. The output signal may be transmitted either to the controlled device or to other logical control functions. The controller compares input from sensors with a set of instructions such as set point, throttling range, and action and then produces an output signal. The control logic usually consists of a control response along with other logical decisions that are unique to the specific control application. The control response conveys as to how the controller functions. Control responses are characterized as: • • • • • •
Two-Position Control Floating control Proportional (P Only) control Proportional Plus Integral (PI) control Proportional Plus Derivative (PD) control Proportional Plus Integral Plus Derivative (PID) control
Controlled Device or Output A controlled device responds to the signal from a controller or control logic in a way that changes the condition of the controlled medium or the state of the end device. These end devices include valve operators; damper operators, electric relays, fans, pumps, compressors, and variable speed drives for fans and pumps applications. Two-Position Control In a two-position control sequence, the controller compares an analog or variable input with instructions and generates a digital (two-position) output. The instructions involve definitions of upper or lower limits. The output changes value as the input crosses the limits. There are no standards for defining limits. Common terms for limits include set point and differential. Set point indicates the value where output pulls-in, energizes, or is true. The output changes back or drops out after the input value crosses through the value equal to the difference between the set point and the differential. Two-position control functions as a simple switch. Two-position control can be used in basic control loops for temperature control or for limit control such as freeze stats or outside air temperature limits. Floating Control A floating control response produces two digital outputs based on changing variable input. One output increases the signal to the controlled device while the other output decreases the signal to the controlled device. The control response also involves an upper and lower limit with the output changing as the variable input crosses these limits.
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FLOATING CONTROL RESPONSE
According to actions of the control systems the controllers are categorized in either Two-position (On/Off) or Proportional (‘P’ action) or Integral (‘I’ action) or Derivative (‘D’ action) and combination of all above, e.g P, P+I, P+D, P+I+D etc. Proportional Action Control Mode A proportional control response produces an analog or variable output change in proportion to a varying input. In proportional control, a unique value of the measured variable corresponds to full travel of the controlled device and a unique value corresponds to zero travel on the controlled device. The change in the measured variable that causes the controlled device to move from fully closed to fully open be called the throttling range. The loop controls within this throttling range, assuming that the system has the capacity to meet the requirements. The type of action dictates the slope of the control response. In direct-acting proportional control response, the output will rise with an increase in the measured variable. In a reverse-acting response, the output will decrease as the measured variable increases. Offset is defined as the difference between the control point and the desired condition. A proportional controller utilizes the principle of negative feedback. The relationship of the gain Kp of the amplifier (the proportional sensitivity) of the control can be obtained by the controller output, which is directly proportional to the deviation (error) of controlled variable (input). The controller output is given by:-p=(100/P.B).e + po,where e is the error, po is the bias or the offset(initial condition is po)and Kp is proportional gain and is equal to 100/P.B.(proportional band, is denoted as P.B, it is inverse of gain Kp) .P.B.is to be chosen such that oscillation is minimum, P.B. is increased to damp the oscillation& attenuate the input cycle. Desired value must differ from the measured value and the measured value is controlled to a proportional desired value by off setting the set value. In proportional action output p = po at error=0 and is constant If the deviation (error) exceeds the Proportional Band then the output may exceed the range of correcting element; thus large gain is required then. In proportional controllers: KORBA SIMULATOR
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i) ii) iii)
The output is a constant equal to output po for the error = zero If there is error, for every 1% of error a correction of Kp percent is added to or subtracted from po, There is a band of error about zero of magnitude PB within which the output is not saturated at 0 or 100%.
In the Op-amp circuit, the resistance R1 varies the proportional const and so the P.B. The electronic proportional controllers are basically the amplifier, which receive small voltage signals and output higher voltage/power signals. The controller output eo = K. (ei –eo.R2/R1) K .R2/R1 > 1 hold good Normally Laplace transform, is obtained for the dynamic response of the controller from transfer functions and the overall transfer function defines the system behavior. Gain G(s) = Eo(s)/Ei(s), Kp= Ri/Ro, Proportional Constant
Prop. Control response
Prp Cont characterstic
Prop. Control set-up
The diagram below gives the arrangement of control system in which the valve responsible for the boiler combustion say oil/coal valve is regulated to control the boiler outlet/steam pressure. The desired steam pressure (Desired Value) is applied to the controller which also receives the feedback signal of actual steam pressure (Measured value), an error is created (e=M.V.-D.V) thus, which is amplified by gain factor K to a value suiting to the valve regulation.
Proportional Control Set up
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Effect of Controller Output for gain K=1
Effect of Controller Output for gain K=2
The controller output varies according to the gain (proportional gain constant Kp=1,Kp=2,Kp=4) as shown in the figure. The effects of gain Kp =1 and the gain Kp =2 has been shown in another sketch from which it can be inferred that the Off-set values (Deviation from desired output) can be reduced by increasing values of Kp but can never be neutralised since the proportional action mode generates Off-set (as is clear from the equation). Since the inverse of Kp is the Proportional Band (P.B), so with Kp=1,P.B=100%and Kp=2, P.B=50% and the pressure oscillates with rise of Kpat reduced offset than before when Kp=1. ‘Kp’ is the gain of the proportional controller, which can be adjusted. System performance of control actions can be predicted on the basis of Step response. Integral Action (Reset Action) Control Mode In integral action controller, the output increases at a rate proportional to the control variable error and so the control output is the integral or the error over time with a gain factor (integral gain) the output p(t) =(1/KI) e.dt and the Transfer function is given by. P(s)/E(s)= Ki/s. When the error is zero the output is fixed at the value that integral term had when the error occurred. The integral time (TI) adjusts integral action while a change in value of the Kp affects the proportional and integral parts of the control action If the error is not zero the proportional term contributes a correction and the integral term begins to increase or decrease the accumulated value depending on the sign of the error and the direct or reverse action. In Integral mode the value of the controller output p(t) changes at a rate proportional to the actuating error signal as given above. For zero (0) value of e(t) the value of p(t) remains stationary. The integral time Ti is adjustable and this Ti affects the integral control action. The integral component removes any standing error as achieved by I action alone; although it effects to sluggish the controller response. The inverse of the integral time is termed as the Reset rate, which is measured in, repeats per minute. In the OpAmplifier circuit, the R (variable) and C decide the integral time and can be set as desired.
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Derivative Action (Rate Or Anticipatory) Control Mode In derivative control (also termed anticipatory), the controller output is directly proportional to the rate of change of control variable;. Since it is time dependant at deviation (error) of zero value the output is zero and there is no output. In Derivative (rate) mode the output p = Kp. KD d ep/ dt where KD = derivative gain constant d ep/dt = rate of change of error. If error changes by an amount ep in a period of time TD, then change in output is equal to change in output as a result of proportional action. The derivative action responds only to the rate of change of error. In the ckt combination of R1& C in ratio of R2 can be set for the derivative time. The graph below shows the effects of modes of controls (P, PI, PID),it is observed that the recovery time in proportional mode if is X then due to addition of Integral mode(in PI) the recovery time of process error is increased a lot say 2 times X, and if further derivative action is added (in PID) , the recovery time of the process error is reduced a lot say ½ X.The Proportional action controller maintains the output at an offset whereas the Proportional plus Integral or the Proportional plus Integral plus Derivative controllers maintain the output at set value line as is clear from the graph.
Curves showing the Proportional, Integral, Derivative Action Control Modes
Proportional Plus Integral Control (Pi) Mode PI control measures offset or error over time. The error is integrated, and a final adjustment is made to the output signal from a proportional part of this model. PI control response will work in the control loop to reduce the offset to zero. A well setup PI control loop will operate in a narrow band close to the set point and not over the entire throttling range. PI control loops do not perform well when set points are dynamic, sudden load changes occur, or the throttling range is small.
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Prop. + Int. response
P+Icontroller O/P for Ramp Error
Proportional + Integral Contr Set up
The proportional gain and the integral action time play important role in (PI) control system; its Output is p =Kp. ep +Kp.KI ep(t) dt and Transfer function is P (s)/ E(s) = Kp(1+1/ TIs) In PI mode one to correspondence of the proportional mode is available and the integral mode eliminates the inherent off set The integral function provides the required new controller output and error goes to zero after load change. The integral term can not become negative thus it will saturate at zero if the error and action to drive the area to net is negative value. The system is useful in frequent and large load changing type processes. The effect of the integral action shifts the whole proportional band. The output saturates whenever the error exceeds the Proportional Band limit. The P+I control set up shown below is similar to the proportional control set up with addition of integral unit. The Step change effect is shown in the curve by which it is clear that the error is almost neutralised due to P+I action and the valve opening is also less fluctuating. The ramp change is also shown and the combined effect is drawn in next curve, which gives that integral effect improves the controller response although process speed is slowed down.
Proportional + Integral Control Set up
P + I Controller Output for Step Error
Proportional Plus Derivative Control (PD) Mode In this mode the cascaded use of proportional and the derivative modes is made. The effect of derivative action is moving the controller in relation to the error rate change. The controller output is given by p = Kp.(1 + . KD d ep /dt) The transfer function of the PD is P(s)/E(s) = Kp (1+TDs)
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Option of proportional controller cannot be eliminated, however it can handle past process load changes as long as the load change off set error is accepted. The setup in electronic circuit has been drawn for the P+D control .a ramp change of the load has been shown from where it can be inferred that with D action the correcting signal is anticipatory in nature and it is created at times when change takes place but in opposite direction of the change process, which is due to the anticipatory characteristic of derivative control.
P + D Controller O/P for Ramp Error.
Electronic P + D controller set-up
Proportional (+) Integral (+) Derivative Control (PID) Mode PID control adds a predictive element to the control response. In addition to proportional and integral calculation, the controller will compute the derivative or slope of the control response. This calculation dampens a control response that is returning to set point so quickly that it would overshoot the set point. PID control should be selectively applied to control loops. The combination of all three modes i.e. the PID mode eliminates the offset of the proportional mode and still provides fast response. Each of the three constants (Kp, KI ,KD) can be varied independently without affecting the effective value of the other two i.e. the integral and the derivative Controller output p is given by P(t) = Kp..ep + Kp.KI ep(t)dt + Kp.KD dep/dt + po
The transfer function of PID controllers is: P(s) = Kp (1+TDs +1/TIs) E(s) The P+I+D control set up has been shown which is further addition of D in the P+I set up. The control characteristic due to result of typical error change is shown .from that it can be observed that the combined effect gives result at very short duration(refer the recovery time been reduced tremendously due to all three control mode action as shown in earlier curve)
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Proportional + Integral Control Set up
P+I+D Controller O/P for Step: Error
Electronic P + I + D controller
P+I+D Controller O/P for Ramp Error
Various modes of controls can be achieved by using Op-Amps, capacitors & resistances. The controller output in proportional mode is P: KP = R2/R1, Proportional plus Integral mode is PI: KI= 1/R2.C (Output inverters are included for eliminating instability and expected fast variable time change). Proportional plus derivative mode is PD: KP= R2/(R1+R3) and KD= R3.C. (Offset error of P action remains and D term provides the rate action) and in Proportional plus Integral plus Derivative mode the gains KP is obtained by R2/R1, KI by 1/RI.CI and KD by RD.CD etc. Bump less transfer: - If p’ is the signal value corresponding to the position value of the correcting element and p is the output signal so p’= p be met by automatic control. In manual control p varies as per deviation (error) and p’ varies independently.. To transfer from manual to auto control, the p’ = p be ensured. If it is achieved; the correcting element will respond and make large disturbances minimized, thereby Bump less transfer will be possible in Manual to auto operation. PI, PD & PID modes of control in Electronic circuits have been shown below:
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DIRECT/DISTRIBUTED DIGITAL CONTROL SYSTEM (DDCS) Direct digital control (DDC) systems consist of networked microprocessor-based controllers connected to analog and digital devices, which either sense information or control components of a system. Control logic initiates and sequences operations programmed with software stored in the hardware. Analog-to-digital (A/D) converters transform analog electrical values into digital information for the microprocessor. The analog sensors may be thought of as transducers that convert a thermodynamic property to an electrical property. Most systems include stand-alone or remote controllers with software to eliminate the need for continuous communication. A personal computer workstation primarily monitors status and stores back-up copies of the programs, records alarms and trending functions, and archives historical information to operate the energy management system (EMS). Relatively complex strategies and energy management functions are normally available at low levels in the system architecture. Electric-to-pneumatic transducers (E/P converters) can provide pneumatic actuation. Controlling a system by DDC, involves three distinct steps: • • •
Measure a variable and collect data Process the data with other information Cause a control action
A control loop comprises three main components: sensor, controller, and controlled device. These three components or functions interact to control a medium. Figure 1 shows air temperature as the controlled medium. The sensor measures air temperature and outputs data; the controller processes the data; and the controlled device causes an action. Similar figures could illustrate, the pneumatic or electronic control systems, where the controller is a separate and distinct piece of hardware. In a DDC system, the controller "function" takes place in software, as shown in figure 2. The double lines indicate the boundary in and out of the DDC controller.
Figure 1. Basic control loop
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Figure 2. DDC control loop
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Algorithms describing the mathematics of the software handle the controller function. Various modes of control have been described earlier in details. However the sensor, points, data classifications etc. have been elaborately explained below. Sensor Sensors measure the controlled medium or other control input in an accurate and repeatable manner. HVAC sensors may measure temperature, pressure, or humidity. Some sensors may measure other relative temperatures, time of day, electrical demand condition, or other variables that affect the controller logic. Other sensors input data that influence the control logic or safety, including airflow, water flow, and current, fire, smoke, or high/low temperature limit. Sensors are an extremely important part of the control system and can be a weak link in the chain of control. Points The word point is a common term used to describe data storage locations within a DDC system. The data can come from sensors or from software calculations and logic. The data can also be sent to controlled devices or software calculations and logic. Each data storage location has a unique means of identification or addressing. DDC data can be classified three different ways-by types, flow, and source. Data Classifications Data types are digital, analog, and accumulating. Digital data may also be called discrete or binary. The value of the data is an integer, a 0 or 1, and usually represents the state or status of a set of contacts. Analog data is represented by numeric or decimal number (usually defining a varying electrical input that is a function of temperature, relative humidity, pressure, or some other variable). Pulse input data are the accumulating data and are represented by a numeric or decimal number; the resulting sum is stored. Data flow refers to whether data go to or from the DDC component or logic. Input points describe data used as input information and output points describe data that are output information. Points can be classified as external when the data are received from an external device or sent to an external device. External points are sometimes referred to as hardware points. Internal points represent data that are created by the logic of the control software. Other terms used to describe these points are: • • • •
Virtual points Numeric points Data points Software points
Global or indirect points are terms used to describe data that are transmitted on the network for use by other controllers.
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Common Point Types Analog input points normally imply an external point and represent a value that varies over time. Common analog inputs for process applications include temperature, pressure, relative humidity, carbon dioxide, or airflow. Analog output points are control signals for modulating valve positions, damper positions, or drive speed. Digital inputs for process applications are usually status indicators, such as whether or not a motor is running, or for fans, pumps, motors, and lighting contactors. A temperature high limit is considered a digital input because, although it is monitoring an analog value related to temperature, the information that is transmitted to the controller is a digital condition indicating whether or not the temperature has exceeded a threshold. Digital outputs usually control relays that command motors or other devices to turn on or off. Software Characteristics There are many different software programs used in DDC systems. DDC systems include operating software, configuration software for the points or system architecture, programming software, and software for alarms. The characteristics of the programming software can be an important difference in various DDC systems. There are three common approaches used to program the logic of DDC systems: • • •
Line programming Template or menu-based programming Graphical or block programming
Line programming-based systems use software languages, similar to Basic or FORTRAN with the process subroutines. Familiarity with computer programming is helpful in understanding and writing logic for any process applications. Menu-driven, database, or template/tabular programming makes use of templates for common process system logical functions. These templates contain the detailed parameters necessary for each logical program block to function. How one block is connected to another or where its data comes from is known as data flow and is programmed in each template. Graphical or block programming is an extension of tabular programming. Graphical symbols connected by "data flow" lines represent the individual function blocks. Symbols similar to electrical schematics and pneumatic control diagrams depict the process. Graphical diagrams are created and the detailed data are entered in background menus or screens. Architecture System architecture is the map or layout of the system used to describe the overall local area network (LAN) structure. This map will show where the operator interfaces with the system and may remotely communicate with the system. The network, or LAN, is the media that connects multiple devices. This network media allows the KORBA SIMULATOR
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devices to communicate, share, display and print information, and store data. The most basic task of the system architecture is to connect the DDC controllers so that information can be shared between them. LAN Communication Communications between devices on a network can be characterized as peer-to-peer or polling. On a peer-to-peer LAN, each device can share information with any other device on the LAN without going through a communication manager. The controllers on the peer-to-peer LAN may be primary controllers, they may be secondary controllers, or they may be a mix. The type of controllers that use the peer-to-peer LAN will vary with different manufacturers. In a polling controller LAN, the individual controllers cannot pass information directly to one of the other controllers. Data flow from one controller to the interface and then from the interface to the other controller. The interface device manages communication between the polling LAN controllers and with higher levels in the system architecture. This same device may also supplement the capability of polling LAN controllers by providing the following functions: Clock function Buffer for trend data, alarms, messages Higher order software support Many systems combine the communications of a peer-to-peer network with a polling network. In this case, the interface communicates in a peer-to-peer fashion with the devices on the peer-to-peer LAN. The polling LAN-based devices can receive data from the peer-to-peer devices but data must flow through the interface. Controller
Classification
Many DDC manufacturers make two distinct levels of controllers. Some make only one. These levels describe where the controller resides within the system architecture on the control network. Knowing the difference between these controllers is important because the appropriate controller is application dependent. Many specifications do not distinguish between the various types of controllers. Higher-end controllers normally reside on a higher-level network and communicate in a peer-to-peer fashion. These are called primary controllers. Peer-to-peer, means that the controllers can share information to other peer-to-peer devices without going through an intermediary device. (Called a supervisory interface). Such controllers have more memory, more sophisticated CPUs, higher resolution A/D converters, more accurate clocks, and can store more complex control strategies as well as trends, schedules, and alarms. Manufacturers also make lower level controllers that normally reside on a lower levelpolling network. These controllers have more limited memory and processing capabilities and must use a supervisory interface device to communicate with all KORBA SIMULATOR
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other devices. There are many different designs. Some are designed for terminal applications like variable air volume boxes or fan coil units. Others may be used for air handling systems with simple to moderately complex sequences of operation. These terminal controllers are usually configured for the number of points required for that application. Some of these controllers use a free form of programming, which requires a complete set of custom programming, while others have application specific programs for typical applications. These programs have selectable parameters that can be set up for each individual application. Since these controllers have more limited memories, they usually do not store historical information (such as trends) and rely on the supervisory interface for this function. The secondary polling networks are configured such that one supervisory interface can monitor a limited number of controllers. This limitation varies by manufacturer. A large number of controllers on a secondary controller network can negatively affect the number of trends that can be practically used, the amount of data that can be processed, and the speed of transmission over the network. How many is too many on this secondary network? This varies and depends on the manufacturer, the speed of their network, and the application in question. In a process control applications, these lower-level controllers may be adequate for many simple systems, but a primary controller is more appropriate for critical applications. How does one specify these distinctly different controllers? First, the engineer must define requirements of various types of controllers and their corresponding interfaces (both network and operator). Once defined, the engineer can dictate which controller should be used on various applications. The process control systems have undergone dramatic changes. They have evolved from pneumatic controls through various generations of EMS and DDC systems to current generation distributed DDC. At this time the computer industry trend of increasing processing power and memory at lower cost is quickly influencing DDC controllers. The advent of open protocols, the increased availability and use of site/building/campus networks, and the interfacing of DDC systems to the Internet have increased the complexity of these systems. During the last 20 years (relatively short time), we moved from a non-proprietary communication protocol to one that has been very proprietary. In addition, the control logic that was distributed to single function hardware components (receiver controllers, and switching relays) now resides in software. These are significant changes to a critical subsystem any process systems, which is vital to the performance and basic operation of industrial units. The DDC system is the "brain" of the process industry dictating the position of every damper and valve in a system and determining which fans, pumps, and auxiliaries run and at what speed or capacity. There is a tremendous need for all involved in the process to increase their knowledge of control systems. Many product lines are presented in generically described layers. Vendors' proprietary controllers and interface devices are placed on these layers, allowing a user to compare similar systems. The user can penetrate this architecture diagram. KORBA SIMULATOR
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The software adopts manual mode when the plant is being regulated manually, independently of the microcomputer. Match mode is adopted for two successive execution cycles, when transferring from Auto to Manual or when BAD data is detected; it allows the software to be ‘initialised’ to the prevailing plant state. Autonormal is the mode adopted when the plant is under unrestrained automatic control. Auto-constraint mode may be adopted when automatic control has to be suspended because a plant limit has been reached. The development of the so-called computer on a chip in the form of a single integrated circuit (IC) microprocessor has given considerable impetus to the use of DDC. Above information provides theoretical background of DDCS used in a small process but in power plant many computers and related systems are grouped to achieve complete plant operation by use of hybrid controls deploying pneumatic/electronic/hydraulic units. Since our simulators are uniquely using Analogue Control System, only preliminary information has been given as above. In another type of Direct Digital Control System (DDCS) as employed by CEGB in their power plants, the computer performs all the functions of error detection and controller action; digital logic circuits are integral part of the loop. DDC have the capacity to control multivariable processes with interactions between elements. The keyboard functions as the communication medium; VDU terminal uses plain language instruction. The computers in DDCS are referred as control centre. The mainframe computer is the central point for accepting information transmitted from the process; in our case the power generating plant. It processes and displays the result to an operator through Data Processing System (DPS). The computer has the potential of loop control. The plant signals are sampled and calculations are repeated at intervals and treated mathematically. The performing close measured values for the control system is derived by processing large number of detectors / sensors of the plant parameters which are separate from the electronic control system In process where hybrid type control is utilized both DDCS and electronic controls are used controlling carry out the programming but since they do not have the information of control engineering in detail it becomes difficult to communicate the control requirement with sufficient precision in programming. As such several commercial hardware and software packages are marketed which use high level languages and the control engineers can write their control software directly; software packages are written suiting to a particular process not particularly for the power plant The minicomputer and microprocessors plus the user-friendly operating systems have been developed which software engineers can utilize relatively straight forward. The fig. Below shows the DDCS logic diagram. The software used by an individual control centre besides in the memory the control centre and all signal associated with that software are connected of to the input / output hardware of the control centre. Presently data processing (DP) is carried out in systems having mainframe computers. The computers used are mostly the microcomputer-using CUTLASS language is typically developed by CEGB, UK for DDC application. KORBA SIMULATOR
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The CUTLASS, therefore, has the facility for carrying out arithmetic (co-efficient, time constants etc.) and logical manipulation from data, including IF statement, SubRoutines and FOR loops for calling up items of data arranged in arrays. All data types can have the status BAD either because they are indeterminate or because the user has used an IF statement to reject unacceptable data item. The CUTLASS DDC language recognizes four alternatives of control mode-manual, match, auto-normal and auto-constraint. The software adopts manual mode when the plant is being regulated manually, independently of the microcomputer. Match mode is adopted for two successive KORBA SIMULATOR
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execution cycles, when transferring from Auto to Manual or when BAD data is detected; it allows the software to be ‘initialized’ to the prevailing plant state. Autonormal is the mode adopted when the plant is under unrestrained automatic control. Auto-constraint mode may be adopted when automatic control has to be suspended because a plant limit has been reached. The development of the so-called computer on a chip in the form of a single integrated circuit (IC) microprocessor has given considerable impetus to the use of DDC. MULTIPLEXING Multiplexing is the process of sharing a single transmission channel with more than one input. There are two main types: i) Time-division multiplexer (TDM) and ii) Frequency-division multiplexer (FDM). Time-division multiplex (TDM) System The TDM multiplexer consists of a number of switches in the transmitter such that each analogue input (V1, V2….Vn) is connected to the transmission channel in turn. If the multiplexer is differential, two switches are provided for each input signal, as actually shown in figure; if the multiplexer is single-ended, only one switch per input is provided and common ground return completes the circuit for all input signals. A control unit whose function is to provide the signals, which select the various switches in the multiplexer and provide timing for the multiplexer and other associated equipment, controls the switches. Often the switches are selected in sequential order under the control of a ring counter circuit in the control unit. A similar set of multiplexer switches is required at a receiver, and the synchronizing signal in the transmitter data is used to synchronize the receiver and transmitter switches so that data channels are isolated. A major difficulty with the TDM system is that each input is measured at a different time, and these values are not valid for comparison if there is appreciable change in magnitude of the inputs between samples. Sample and hold units are used to overcome this problem. Each signal input to the TDM system is connected via a sample and hold switch to a capacitor. All inputs are sampled at the same time by closing these switches, and when the switches open the input voltage are held on the capacitors. The multiplexer can now select each hold input in turn. The analogue switches maybe electromechanical or solid-state switches. Electromechanical devices such as dry-reed and mercury-wetted contact relays have sampling rates generally limited to about 250 samples per second. Solid-state switches such as bipolar or field effect transistors (FET’s) can be used at sampling rates in excess of 100 000 samples per second. The limiting factor to sampling rates is the presence of switching transients. The switch characteristics can contribute errors in the signals; thus leakage current, bipolar transistor ‘on’ offset voltage and FET ‘on’ resistance can cause error. In theory, the sampling rate required to accurately transmit a signal whose highest KORBA SIMULATOR
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frequency component is fHz is 2f samples per second. In practice, due to limitations in the reconstruction filters, it is necessary for the sample rate to be, say, 5f samples per second. Thus, for example if f=10 Hz and 100 data channels are to be sampled, the sampling rate must be at least 5000 samples per second. The time between samples in 200 µs, & each input is read at intervals of 20 ms. Generally TDM is used for may channels of data containing low frequency information as obtained from temperature, pressure, loading strain and voltage monitors.
Frequency-division multiplex (FDM) system A schematic diagram of AM frequency-division multiplex system is given above. Each input modulates an assigned subcarrier at comparatively low frequencies (540kHz). The AM-FDM system permits a number of data signals to be simultaneously sent over a common transmission channel. Data usually are sent by a FDM are vibration, acoustic noise, acceleration and dynamic strain. Such a system is well adapted to analogue signals, as all input and output voltages are continuous, whereas a TDM system has discontinuous outputs and is well adapted to digital signals, especially when sample and hold circuits are used. KORBA SIMULATOR
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PROGRAMMABLE LOGIC CONTROLLERS (PLC) The Discrete state relates to each event in the sequence and specified conditions of all operating units of the process. Set off conditions is described as a discrete state of the whole system. An event in the system is defined by a particular state of the system. An event lasts for as long as the input variable remains in the same state and the output variable is left in the assigned state. In a discrete state process each event is described by a unique specification of the hardware& it must be carefully defined in terms of the nature of two states & its relation with the process. The sequence of events can be described in narrative fashion (flow-chart, or Boolean equations) SWITCES are the primary input elements in the discrete-state control system; Normally Open (NO) or Normally Close (NC) states are activated from many sources. Different types of switches are symbolized as shown in logic diagrams. Ladder diagrams are used for designing and describing the sequence of the process events. Computer based method of control also termed as programmable logic control uses relay control system and ladder diagrams. There are only discrete number of possible states of input and output values e.g. for three input and three output variables 26=64 possible states shall be there. In a process control the discrete state situations are involved in operations some of these are in series/parallel. Some of the events involve the discrete setting state in the plant e.g. valve open or close, motor on or off etc. Other events involve regulation of continuous variable over time or the duration of an event e.g. maintaining temperature at a set point for a given time. Continuous control for controlling flow of liquid through a valve into the tank and some unspecified flow out of the tank can be referred as an example of continuous variable regulation. The controller operates according to some mode of control in order to maintain the level against variations induced from external, Thus if outflow increases the control system will increase the opening of the input valve to compensate by increasing input flow rate and the tank level is regulated. Here both levels and the valve setting vary over a range. Even if the controller is operating in on / off mode there is still variable regulation although the level will now oscillate as the input valve is opened and close to compensate for output flow variation. If the same example is studied in discrete state control logic then the level and valve setting are discrete because they take two values. This means that the valves can only be opened or closed and the level is either above or below the specified level. There is no continuous measurement or output over a range because the variables, level measurement, input valve setting and output valve setting are of two state quantities and these form the discrete state control. In PLC system the sequence of event must be described that will direct the system through the operations to provide the desired end result. Narrative statement as to what must happen during the process operation and what events must occur to achieve the object is required for such requirement. In systems, which run continuously a start up, or initialization phase and running phase are typically there it is important to realize that with relay. Control each RUNG of the ladder is evaluated simultaneously and continuously influences KORBA SIMULATOR
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consequences are immediate which is not there in the computer based programmable controllers; the relays are replaced by software, that are able to accommodate and make changes in programmed sequence.
Time delay actions and counters, solid state devices for controlling high power AC/DC in response to low level commands by use of SCRs and TRIACs are part of the PLC/software. The processor (being the main hardware of the computer) executes programme to perform the operations according to ladder diagram or binary commands, performs arithmetic and logic operations on input variables, functions under a permanent supervisory O.S for directing the overall operation from data input / output and executes the user programmes. In PLC the processor is a serial machine, which performs one operation at a time and sequentially sample input to evaluate the required programme. Another main hardware of the PLC is the input module, which examines the state of physical status and other output devices to put their state into a suitable form acceptable to the processor. It converts the input into binary 1 or 0 state. The other main KORBA SIMULATOR
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hardware of the PLC is the output module. The output modules supply AC power to external devices viz., motor, light, solenoid etc. at suitable driving power. It also accepts binary 1 or 0 input from the processor and uses this to turn on or off the output devices. The PLC operation considers the simultaneous selections of ladder diagram and relay sequencers in two modes, the input scan mode and the execution mode. The scan time is around 20 mS and it depends upon the clock frequency of the processor. Programming unit, RAM, ROM, time relay & counters, programming diagram interpretation, addressing and programming units are important hardware in the PLC. As the technology of computers and PLC advances, many new features are included such as computerized control of continuous processes and DDC as a builtin feature of the PLC. The ladder diagram includes rungs for such control system with specified proportional, integral, derivative gains. ANALOGUE CONTROL SYSTEM (ACS) The Analogue Control System (ACS) has been supplied by M/s. H & B, West Germany. Power supplies to transmitters are derived from various supply modules placed in the cubicle. Eight no. of cubicles cater the total requirement of ACS. All the control modules have the facility of galvanic isolation of the order of 250 volt between input and output terminals. Transmitters output is to 4 to 20 mA or –2V to 10 V as per the requirement. Electronic modules e.g. AV95 (Power Supply), AW02 (Galvanic Isolation), AV96 (Power Supply), XU01 (Computing), XP01 (Comparator), XK12 (Limit Value), XN01 (Max./Min Selector), RS01 (Controller), XM02 (Analogue Memory), HA01 (Interlock Selection), HS01 (Relay Module), RL01 (Controller), RK01 (Controller), RK04 (Low Range Controller), XM13 (Memory), LL02 (And Logic), LL03 (Logic Unit), VV01 (Signaling), LT01 (Relay Unit), HV02 (Amplifier) etc. have been used for various application in the ACS system. The detailed testing, checks, operation, maintenance can be referred from the C&I manuals on the above systems. Establishing Automatic Operation requires certain steps like operate the plant such that the measure variable is within the limits to that of the set values, ensure the plant stability and safety prior to switching over to automatic operation, record the plant parameters before automatic operation is switched, connect the recorder and assign the correct colour for recording the measured, desired value etc. Operate the manual switch and regulator such that the position follows demand at the appropriate speed when a demand changes made. Record the final setting of the controller in tabular form. For more details refer the designing of signal conditioning as given in earlier chapters. In 200MW Thermal Power Plant Korba, following controls have been incorporated. • •
Co-ordinated Master Control (Firing Rate; Fuel flow & Air flow). Drum level & Feed regulating station differential pressure control
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• • • • • • • • • •
Feed pump re-circulation flow control Dearator Level & Pressure control. Hot well level & Re-circulation control S/H Steam & Reheat steam temperature control. Heavy fuel oil control. Furnace pressure control Air flow /F.D.Fan control. Primary Air Pressure Control. Pulverisers A, B, C, D, E, F, Temperature / Air flow control. Mill Feeder A, B, C, D, E, F, Speed control.
CO-ORDINATED MASTER CONTROL (CMC) OF THERMAL POWER PLANT The power flow in the grid from various generating units needs to be controlled and regulated as per the consumer demand and the capability of generating units. Net power flow in the grid is certainly zero for maintaining the energy/heat throughput to the generating units and energy/power sent out to the system. This results complete energy balance. As the boilers at the thermal generating units supply steam to the Turbo-generators, the balance of heat input to boilers with that of the power generated by the TG set need to be regulated & controlled by grid managing load dispatch center authority (LDC). If power is generated as per the instructions of the LDC within the limits of generating capability of each unit then the economic and efficient loading of the boilers can be ensured. Let us look at to the block diagram of CMC we find that in addition to LDC, the automatic operation of power plant is linked with the load demand, frequency correction, load rates, load limits, unit capabilities (Runback limits), turbine load index, throttle pressure control, turbine generator control, boiler master (Fuel & Air flow control). Feed water flow control is not directly linked with the CMC (as shown in block diagram) in our 200MW sets as the three-element control of Boiler drum control fully takes care of steam flow/load index. Similarly the Gas flow control is delinked in our plant, as actual value signal is furnace suction to wind-box Diff Press as explained in I.D.Fan control loop.
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BLOCK DIAGRAM OF CO-ORDINATE MASTER (INTEGRATED) CONTROL
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The Co-ordinated Master Control System (CMC) operates in four modes as given below. •
Co-Ordinated Mode
•
Turbine Follow Mode
•
Boiler Follow Mode
•
Runback Mode
CMC- VARIOUS MODES SELECTION BY SWITCHES
CO-ORDINATED MASTER CONTROL MODE The objective of the CMC is to enhance the response of the boiler and turbine control systems under all operating conditions while maintaining the outputs of the turbine, boiler and all major plant auxiliaries within the safe operating limits. The CMC receives unit megawatt demand signal from the load dispatch centre & throttle pressure signal. The unit master station translates the load demand signals within maximum / minimum limits as per unit requirement for the boiler and turbine control system. The generated load is controlled through regulation of turbine control valve by EHC signal as received from GNI output through selectors explained KORBA SIMULATOR
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in CMC loop details (as described in pages that follow). The functioning of CMC in general & in nutshell can become clear from the block diagram given below.
BLOCK DIAGRAM OF COORDINATOR MASTER CONTROL
TURBINE FOLLOW MODE In the turbine follow mode, Boiler is manually controlled from the Boiler Master A/M station and the turbine remains in initial pressure mode controlling the throttle pressure. In turbine follow mode, the actual unit load as selected by energizing of relay/switch T1, becomes unit load demand signal and the unit load demand signal as generated from Unit Master/ALDC gets by passed. In the turbine follow mode, the steam flow to turbine becomes the demand signal for boiler master. (since the steam flow to turbine is measured by using a computed signal of turbine first stage pressure and main steam temperature corrected signal as obtained from curve plotter ; there is no direct measurement of steam flow). The Boiler master station also receives the error signal from steam throttle pressure comparator (actual – desired pressure; this difference is kept within -+ 3 kg/cm 2) and the boiler master outputs the signal that is termed as firing rate demand signal to the fuel and air control loops as required for efficient boiler operation and pressurizing. The turbine load is controlled through regulation of turbine control valve by EHC when it is operative/selected in Load control mode and also the CMC has been selected to modify the turbine load. Block diagram shown below is clearly giving turbine/ boiler operation considering the reference signals i.e boiler is controlled by the throttle pressure and the turbine is controlled by its own output i.e. megawatt signal.
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BOILER FOLLOW MODE In Boiler follow mode the Throttle pressure is maintained constant, when put in auto mode of operation and the turbine remains manually operable in limit pressure mode through load controller. In Boiler Follow Mode the throttle pressure deviation becomes the demand signal of the boiler-firing rate through boiler master selected in auto mode. In Boiler follow Mode the actual unit load as selected by energizing the relay T4 becomes the load demand by passing the Unit Master load demand signal and the guided tracking integrator( GNI) sends this signal for further processing and outputting for boiler control. The load signal derived from the export bus is used for controlling the turbine control valve movement. (Refer the block diagram BD-2 for details), thus the turbine is manually controlled through the load controller and in Limit pressure mode. Block diagram shown below is clearly giving turbine/ boiler operation considering the reference signals i.e boiler is controlled by the megawatt signal and the turbine is controlled by the boiler’s output i.e. throttle pressure.
TURBINE FOLLOW MODE
BOILER FOLLOW MODE
RUNBACK MODE The unit capability in terms of load generation is calculated by the run back condition based on the availability/healthiness of the various plant auxiliaries. Each one of the major auxiliary viz; ID Fan, FD Fan, BFP, CEP etc. contribute to the unit loading and the contribution percentage of each auxiliary has been pre-decided thus due to non-availability of the particular auxiliary, the load of the unit shall be reduced equal to the contributing capacity of the particular auxiliary. Let us consider the situations that, if two no. of FD fans are required for full 210MW operation of the unit and if one of the FD fan is tripped or not available due to some major problem, then the unit load shall be brought down to say approx.140MW since available one FD fan can support unit operation for this load, similarly if one mill is not available because only three mills out of 4 mills are required for full load operation then the unit load shall be reduced to 155MW i.e. 210 – 55 = 155MW etc. The unloading rate (run back limit) means the percentage at which the load set point has to be reduced due to non-availability of a particular auxiliary. This unloading rate is set by potentiometers mounted on the electronic module HA-02 and through the minimum selector electronic module XK-01. The load demand of the unit is another signal available to this minimum selector. Hence the output signal from the XK-01 becomes the load set point guidance signal designated as down gradient (GD) of the GNI.
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Further the unit load rate setter outputs, the signal as desired for setting the loading rate and is compared with the lower load margin (decided by TSE) and the minimum of above two signal is fed to the maximum selector XN-01 which also receives the run back rate (unloading rate of the unit) and the maximum of the two is fed to GNI in subtractive port and another parallel signal of the unit load rate is compared with upper load margin (decided by TSE) and the minimum of these two is fed to additive port of the GNI. (Refer the block diagram BD-1 for details). The run back requirement of the unit is suitably processed at the GNI as described above. In the Runback mode, the automatic feed back (GNI tracking) signal to GNI bypasses the load limit selectors & the boiler master PID Controller because of changed contact of relay T2 as the relay T2 has been energized due to selection in runback mode and the boiler firing rate signal from GNI directly controls the boiler operation as per the run back requirement explained above.
COORDINATED MASTER CONTROL (CMC)
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COORDINATED MASTER CONTROL (CMC): detailed loop description A coordinated master panel insert is provided as a means for operator to supervise the operation of coordinated master control system. This panel contains the following items & functions: • Unit master station • Max/Min load setters • Unit load rate/change setter • Boiler master A/M station • Throttle pressure set point • C.V. Correction setter Selection push buttons for boiler follow, turbine follow and coordinated mode •
Indicators for Turbine on CMC, Limit Pressure, Initial Pressure, Run Back
•
Digital indicators for Demand Load, Generated Load. The following interconnection signals between Boiler Control and turbine control are provided to ensure proper functioning.
•
Analogue Signal: Actual load, Frequency deviation, Diff. Temperature, Limit Pressure, Initial Pressure, Diff.Temperature (U min, Lt min), Actual Pressure. Deviation Wp. Reference load. Binary Signal: Coordinated Mode, Turbine follow mode, Boiler follow mode Turbine on CMC, EHC fault.
The Co-ordinated Master control (CMC) has been provided to simultaneously control both turbine and boiler in cohesion with grid demand. The Unit Master station with provisions of auto / manual selection, is the main module in the CMC loop. The Unit Master Station receives the Mw load demand of the unit from the automatic load dispatch centre (ALC) also termed as load dispatchers’ demand in the form of Coordinated Master Control (CMC Load demand) and Unit master station outputs the CMC load demand when this is selected in AUTO MODE operation.
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CMC- FORMATION OF LOAD INDEX AND SIGNAL DISTRIBUTION
Unit Master Station, when put in manual mode of operation, outputs the Mw load demand signal, as generated by/ desired by the unit operator suiting to the plant requirement. Guided Target Integrator (GNI The Guided Target Integrator (GNI) a computer module computes the target signal, the frequency correction signal, the desired unit load rate, as compared with upper and lower power margin generated from turbine stress evaluator (refer TSE chapter for more details). The unit load demand signal as explained above becomes the target value (Zo) of GNI; in addition a fixed 15% (K. δF) signal is added directly to the output of the GNI. For K. δF within the band of +/-15% the correction is added to the GNI output (Ao). This signal (Ao) is subjected to a MAX/MIN cut off by the presets load limits and minimum cut off by the unit capability signal decided solely on the healthiness/availability of the coal pulverisers, PA fans, ID fans, CEP, BFP etc. The unit capability signal forms the Run Back Limit signal for controlling the unit load at times when full no. of auxiliaries are not available. Unit load rates, upper/lower load margins, runback rate, all modify the GNI output as explained in runback mode. Three selectors/ switching relays T1 , T2 and T4 are energized for running the controls in boiler follow, turbine follow, or coordinated mode of operation.
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CMC-GUIDED TARGET INTEGRATOR INPUTS & LOAD DEMAND
The frequency correction signal ensures frequency deviation K. δF > 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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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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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.
KORBA SIMULATOR
342
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
344
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 =
KORBA SIMULATOR
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.
KORBA SIMULATOR
346
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.
KORBA SIMULATOR
347
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
