KORBA SIMULATOR

1

KORBA SIMULATOR

2

KORBA SIMULATOR

3

FOREWORD

Power is the most vital necessity for industrial and economical growth of any nation. Electricity can bring sea changes in quality of life of its society members. NTPC in its endeavour for becoming most significant entity once again after 30 years of untiring and relentless efforts, reaffirm its commitment towards making India a self-reliant nation in the field of power generation. Having proven excellence in Operation & Maintenance of 200 and 500MW units; for the first time we are going ahead with the commissioning of 660MW units at our Sipat Project. This is a major step towards technological advancement in power generation. In the present time, efficient and economical power generation is the only answer to realise our ambitious plan. It is the need of the hour that available human resources who are the at the whelm of the affairs managing the large thermal power plants having sophisticated technology and complex controls, is to be properly channelised and trained. NTPC management firmly believes that skill and expertise up-gradation is a continuous process. Therefore, training gets utmost priority in our company. Power Plant Simulators are the most effective tools ever created. This has computer based response, creation incorporating mathematical models to provide real time environment, improves retentivity and confidence level to an optimum level in a risk-free, cost and time effective way.To supplement the hands-on training on panel and make the training more effective an operation manual in two volumes has been brought out. The operation manual on 500MW plant provide the information comprehensively covering all the aspects of Power Plant Operation which can be useful for fresh as well as experienced engineers. It provides a direct appreciation of basics of thermal power plant operation and enables them to take on such responsibility far more sincerely and effectively. I am pleased to dedicate these manuals (volume- I & II), prepared by CSTI members which is a pioneer institute covering more than 7000 participants till date, to the fraternity of engineers engaged in their services to power plant. The volume-I deals with the Plant & system description and II covers the operating instruction in a lucid way. I sincerely hope that readers will find these manuals very useful and the best learning aid to them. I believe that in spite of all sincere efforts and care of faculty members & staff, some area of improvement might have remained unnoticed. Hence, your valuable suggestions and comments will always be well received and acted upon.

( A. CHAUDHURI )

GENERAL MANAGER

KORBA SIMULATOR

4

KORBA SIMULATOR

5

CONTENTS

CHAPTER NO. TOPIC PAGE NO

1. PLANT SIMULATION AND DATA

ACQUISITION SYSTEM

7-17

2.

BOILER AND AUXILIARIES

19-120

3. CONDENSATE AND FEED

WATER SYSTEM

121-174

4. CONDENSER AND EVACUATION

SYSTEM

175-188

5. HP AND LP BYPASS SYSTEM 189-209

6. STEAM TURBINE AND AUXILIARIES 211-244

7. TURBINE GOVERNING SYSTEM 245-293

8. AUTOMATIC TURBINE TEST 295-319

9. TURBINE STRESS EVALUATOR 321-334

10. GENERATOR, ITS AUXILIARIES AND EXCITATION SYSTEM 335-393

KORBA SIMULATOR

6

KORBA SIMULATOR

7

PLANT SIMULATION AND

DATA ACQUISITION SYSTEM

KORBA SIMULATOR

8

KORBA SIMULATOR

9

PLANT SIMULATION AND DAS THE PLANT SIMULATION The 500MW-training simulator is a complete full scope replica of the 500MW coal-fired unit-6 of Singrauli plant of NTPC, which creates the real time effects of the plant operating conditions on the Unit Control Panel equipments. The actual plant, the equipments, the control systems - all are replaced by their mathematical models and made to run through a real -time execution process of a computer to represent the exact plant dynamics through its process parameters on the Unit Control Panel. THE SIMULATOR SYSTEM ARCHITECTURE THE HARDWARE: - The simulator system is having the hardware organisation as per fig.1

FIG-1 SIMULATOR HARDWARE ORGANISATION UPS System: - It is a 55KVA UPS with 100 % stand by capacity, consisting of Rectifiers, Inverters, Batteries, Stabilizer, Static By-pass Switch, AC distribution panels, etc. It provides regulated power supply to the complete Simulator equipments. The UPS is Supplied by M/S AEG , West Germany.

Computer system and peripherals: - The computer systems supplied are 32 bit digital computers of Encore, USA. The supplied model 32 / 67 is ideally suited for the real time simulation applications. The system mainly comprises of

• Two computers for simulation of plant equipments (SIMULATION COMPUTER)

• One computer for simulation of DAS tasks (DAS COMPUTER)

• Shared memory systems (for coupling DAS and SIMULATION COMPUTERS)

KORBA SIMULATOR

10

• Various peripherals such as magnetic tape drives, disc drives, floppy drives, ‘system consoles, hard copy printers, line printers, Graphics systems (colour monitors /controllers) video colour printer etc.

• Set of cables for interconnecting the system and peripherals.

• The computer system is based on a high speed synchronous bus (called as

SELBUS) , on which the CPU and / or IPU are residing . It supports upto 16 MB main memmory, Input Output Processor (IOP) and peripheral controllers. It offers 18 Selbus slots and four MPbus slots and peripheral space. This system accommodates 800 / 1600 / 3200 bpi streaming Mag tape units and over two Gigabytes of disk storage.

Control Panel: - a Simulator control panel with mounted instruments is replica of Unit -6 of Singrauli Power Plant and is the main hardware of this Simulator. It comprises of UCB section 1 to 3 and CSSAEP panels. Instruments mounted on these panels represent the operation of the real plant processes which are simulated by the computer systems and the computed information is transmitted to these instruments via Input / Output system. In addition to various monitoring and recording equipments, the panels are also equipped with control switches, indicating lamps, annunciation system and DAS system.

Interface (Input / Output) System: - The I/O sub-system forms the interface between the simulation computers and the UCB panels. The main function of the I/O sub system is to update the UCB output points with the current simulated value and to report the state of the UCB inputs to the simulation computer. I/O sub-system consists of four SIMTROLs catering the all sections of the UCB, associated Control Room Equipment (CRE) power supply and special device interface modules. Instructor Station: - Instructor station hardware comprises mainly of Instructor station console and peripherals such as two monitors with keyboards, one video hard copy printer, one remote control unit and one special function keyboard with back lighted push buttons for activation of desired function. With the help of remote control unit, certain functions can be initiated/stopped during training session without the notice of the trainee and training session transients can be hard copied on video printer for further analysis. Data Acquisition System (DAS): - DAS comprises of three color CRTs mounted on UCB-2 panel having assigned as Utility, Alarm and Operation CRT. One additional CRT is also provided on Operator‘s desk. For documentation purposes Hard Copy

KORBA SIMULATOR

11

printers are provided for Alarm and Utility CRT and a Logging line printer for massive and fast documentation. Hard copy of the information from any of the DAS CRTs can also be obtained on video printer through selector switches. THE SOFTWARE: - The Simulator system is having the following software organisation as per Fig.2

FIG-2 SIMULATOR SOFTWARE ORGANISATION Computer operating System MPX-32: - The computers work on a Mapped Program Executive (MPX-32) disk-oriented, multiprogramming Operating system, that supports concurrent execution of multiple tasks in an interactive, batch and real time environment. MPX provides memory management, terminal support, muliple batch streams and intertask communication. It supports 16 MB physical memory address space. An intergrated CPU scheduler and a swap scheduler provide efficient use of main memory by balancing the task based on time distribution factors, software priorities and task state queues. Simulator Control & Executive System Software UNISYSTEM: - UNISYSTEM is a Software tool for use in the developement of large-scale real time application programs. It provides:

• A data base to record and describe the variables, arrays and subroutine used in a program.

• A Modified FLECS compiler that is linked to the database to verify the

legitimacy of variable, arrey and subroutine names encountered in the code being compiled.

KORBA SIMULATOR

12

• A data base manager program to handle the declaration of new variable, arrey, and subroutine name. It also creats COMMON and EQUIVALENCE statements needed to use the variable, arrey and subroutine names in programs.

• A real time program scheduler to execute users programs on a real time basis.

• A plotting program to display results obtained from execution of user’s programs.

Application Software for Plant system Simulator: - The total power plant system is broadly divided into the following subsystems for math modeling purpose:

1. Boiler and Flue gas subsystem.

2. Boiler Water and Steam subsystem.

3. Fuel subsystem.

4. Condensate subsystem.

5. Feed Water subsystem.

6. Turbine subsystem.

7. Electrical subsystem. Each of this subsystem is subdivided into Process interlock and control models based on nature of the model function. These mathematical models are developed based on physical laws of conservation of mass, energy and momentum. The above mathematical models, converted in to the form of simulation software models, are then integrated in a sequential manner to represent the power plant dynamics in totality during all plant operating conditions including pre start-ups checks, preheating, start-up (cold, warm and hot), shut down, power maneuvering, normal operation and specified emergencies. The extent of plant simulation is thorough enough to support the plant operators (the trainees here) to fully participate in plant status evaluation, actual plant operation and control of unusual transients. Application Software for Data Acquisition System (DAS): - Plant computer functions provided by actual plant computers have been duplicated in the simulator. These are

KORBA SIMULATOR

13

• Alarm monitoring of analog and digital input signals and indication of abnormal plant operating conditions.

• Analog trend recording of operator selected analog inputs.

• Logs such as hourly log, turbine run-up log etc.

• CRT displays for analog, digital plant signals and group point displays, alarm

displays, etc.

• Performance calculations. Simulator Instruction Station Software: - Instructor station software is provided with facility for monitoring, controlling simulator conditions and monitoring operator (trainee) actions. It has provision to select all initial conditions and malfunctions and the ability to manipulate external parameters. Interface (Input/Output) System Software: - The I/O system application software consists of tasks running on Simulation Computers and on SIMTROLs. The tasks running on Simulation Computers perform: -

1. Input-Output Transmitting.

2. Misaligned switch checking.

3. Daily Operational Readiness Test. The tasks running on SIMTROLs perform:

1. SEL Interface.

2. Input Transmitting.

3. Analog Output Updating

4. Digital Output Updating

5. Table Management

6. Watch Dog

7. Digital Input Scanning

8. Analog Input Scanning

KORBA SIMULATOR

14

9. Analog/Digital Output Driving, etc,. TRAINING FEATURES The following features of the simulator facilitate a very effective training to the power plant operators: - Initialisation: - The simulator can be initialised to any one of 60 plant conditions from where the training session can start. The instructor can choose the status / conditions of the running simulator (i.e., the plant) and save them as ICs (Initial Conditions) through a special utility software at Instructor Station . A maximum number of 60 such selected plant conditions can be kept stored. Later on , any one of these stored conditions can be retrieved as an Initial Condition and the training session can be started from that plant status . Thus the Initialisation facility provides the flexibility in training by starting the session from any one of the 60 stored plant conditions as per the requirement / level of the trainees and saves time by eliminating the repeated exercise to bring the plant to the required condition again to start with. Freeze/Run: - The FREEZE feature helps the instructor to “freeze” the plant simulation and thus to bring the plant dynamics to a standstill condition.The plant operation can be subsequently resumed from the last frozen status by using “RUN” command by the instructor. When the FREEZE command is issued from the Instructor Station, the simulation software under execution is stopped and the updation of the simulation variables are suspended thereby creating an effect of freezing of the dynamic plant condition. This facilitates the instructor detailed explanation on that particular stage of operation without allowing it to go unobserved by the trainees on the panel. Backtrack: - This facility enables the plant simulation status to traverse back all events of operation for the past 60 minutes. The simulation data is continuously saved for a period of 60 minutes at the interval of one minute each as 60 disc file records. Thus at any point of time, 60 data sets are available representing the plant status for the preceding sixty minutes. The instructor can bring the simulator to any of these last sixty plant conditions by BACKTRACKing to the desired problem time or by BACKTRACKing step-by-step from the present 1st record. Which is the current one saved. If required, simulation session can continue from this backtracked record status to facilitate repeated panel operation or to offer detail explanation to the trainees. Snapshot: - This feature enables the instructor to “SNAP” the plant status as a complete record of all the simulation variables that represent the plant dynamics at the time of snapping. These SNAPSHOT records, as disk files, can be saved with identifying title, date & time and can be retreived any time in future as Initial Conditions to commence the training session from that snapped plant condition. A total number of 60 SNAPSHOTs can be saved and stored as Initial Conditions providing wide range of flexibility in training. Slow Time Mode: - This features enables the Instructor to slow down the dynamic simulation to ten times slower than the real time. Thus in a SLOW MODE Simulation, a trainee can observe the fast transients or certain critical operations more precisely in

KORBA SIMULATOR

15

order to analyse the dynamic behaviour and study the sequence of events thereby enhancing their knowledge and experience. Fast Time Mode: - In this mode of simulation, certain time consuming plant operations like turbine soaking, boiler heating, raising of condeser vacuum, furnace purging, etc. are made to run ten times faster than the real time. Thus the instructor can save the valuable panel time by attenuating the time taken in accomplishing lengthy plant operation stages and offer the saved time to the trainees for better utilisation on the panel. Malfunction: - This is the most valuable feature of the Simulator, which offers the trainees a unique scope for experiencing a large number of malfunctions that occur in a power plant. The instructor can introduce malfunctions in single number or in groups (the selection being dependent on the status of the plant) to simulate the real emergencies as faced by the operators on panel. The trainees are thus given opportunities to tackle those malfunctions by taking suitable corrective operation steps, which are, otherwise, rare events in an actual plant. A total number of 270 malfunctions are available characterised into two types:

1. The Event type malfunctions: These can set the equipment/component failure at an optional pre-selected time.

2. The Severity type malfunctions: These can be started at an optional pre-selected

time with the degree of severity (0-100%) and the gradient (time to reach that severity effects) choosen by the instructor.

The malfunctions available can be selected, activated and reset (cleared) by the instructor without any intimation to the trainees on the panel, which supports a realistic operational environment. Record & Replay: - The RECORD feature enables the instructor to record the training session under progress for a period of two (2) hours. All the changed inputs from the panel and the IS are recorded alongwith time on specified disk files. Maximum 4 nos of records, each of two hours duration, can be stored. The storage can be initiated at any instant of time. The REPLAY feature enables to replay the panel status as recorded earlier. Thus the trainees can observe their previous performance on panel alongwith the instructor’s explanation and analysis. Any of the four-recorded sessions can be selected for replay. Both RECORD & REPLAY functions can be paused and stopped in between. Remote Function: - This function facilitates the instructor to perform any remote (field/local) operations, which are not carried out from main control room. The manual operations of local equipments (e.g. F.O. pumps, valves, isolators, station supply breakers, etc) are simulated from the instructor station for providing necessary permissives and also for controlling process parameters. Crywolf Alarm: - This feature enables the instructor to create false alarms by flashing windows and by making audible cry wolf alarms even though such conditions do not

KORBA SIMULATOR

16

exist in the plant under operation. With this facility trainees’ immediate response/reflexes can be tested. At a time, upto 16 numbers of alarms can be set and reset selectively by the instructor. Override Panel Device: - Any device on the control panel can be OVERRIDEN with a given value (for analog variables) or with a given status (for digital variables) by the instructor at an optional preselected activation time. The value/status of the overridden device remains constant until it is reset to normal operation or overriden with a new value/status. A total number of 32 devices can be selected for overide at a time. The instructor can thus create maloperation of the instruments on the panel to test the trainees undergoing session. External Parameters Manipulation: - This feature enables the instructor to change the values of certain parameters that are not simulated in the software but affect the plant performance. External parameters (inputs) like the grid voltage, grid frequency, calorific value of fuels, etc can be assigned new values. The change will be achieved gradually within one minute. These inputs change the plant dynamics and performance. Thus it offers the trainees scope of plant operation under different conditions on a single platform. Analog Output Reallocation: - Any analog output of the plant simulation can be reallocated to any other meter/recorder on the panel. This permits the instructor to continue the training in the event of some instrument failure on which some important parameters are displayed/recorded. Parameters Monitoring: - Trends of important plant parameters (simulated variables in engineering units) can be monitored on the instructor’s dedicated console to check the trainee’s performance on control panel for the duration selected. The instructor can also change the higher and lower limits of the parameters selected during trend display for better resolution in monitoring. A total number of 80 parameters can be selected, deleted, and stopped for monitoring by the instructor to match his requirements. Limits of the selected parameter can be modified for better analysis. Trainee Test: - This feature is the unique facility in simulator training by which proficiency of operation personnel can be evaluated by the computer. The instructor can assign the trainee a task on the panel and monitor his/her capacity to control important parameters of the plant with a final assessment printout result if opted for. At a time, maximum four nos. of tests can be conducted in parallel depending upon the plant conditions and the tests selected. Each test has the following facilities to be selected.

• Identification of the test by Instructor’s name, Trainee’s name & Exercise number/Title,

• Monitoring or Evaluation type,

• Duration of the test (Run time),

• Displaying the test parameters with updation by every one second,

KORBA SIMULATOR

17

• Trending of test parameters,

• Deleting test parameters already selected,

• Changing of Hi & Lo limits of the test parameters to monitor within a narrow range.

• Displaying of the test results on the Video Monitor.

• Printing of the test results to get a hard copy.

• Thus the trainees can get a feedback on themselves after completing the test

program.

KORBA SIMULATOR

18

KORBA SIMULATOR

19

BOILER AND

AUXILIARIES

KORBA SIMULATOR

20

KORBA SIMULATOR

21

BOILER AND AUXILIARIES SALIENT FEATURES OF 500 MW BOILER. With increase in demand of power in India, new power projects are being constructed with higher capacity and advanced technology for the better economy and reliability of operation. Compared to other lower capacity Boilers supplied by BHEL, these 500 MW capacity boiler have incorporated certain special technical features which are detailed here under: - CONTROLLED CIRCULATION SYSTEM This is achieved by three numbers of glandless pump and wet motor installed in the downcomer line after the suction manifold. These pump motor assemblies have single suction and double discharge introduction of these pumps in the boiler system have led to the designing of a furnace with lesser diameter tubes and high parameters operating characteristics. The advantages of the controlled circulation boiler over natural circulation boiler are given below: -

• Uniform drum cooling and heating. In controlled circulation boilers this is possible because of arrangement of relief tubes inlets to the drum and the internal baffles of the drum from both sides. The internal base plates are arranged in such a way that it guides the steam water mixture from the relief tubes along the whole circumference of the drum. The drum is therefore uniformly heated and cooled.

Whereas in Natural Circulation Boiler, the arrangement of relief tubes and baffle plates is only on one side of the drum and this imposes a constraint on uniform heating of drum. Similar arrangement of relief as in controlled circulation boiler does not exist in natural circulation (NC) boiler because in that case the relief required to be taken over the drum and fed from both sides. This shall increase the pressure losses in the riser tubes and also the hot static head requirement for start up. Since the available head in NC Boiler is very less; efforts are always made to reduce the pressure loss and improve the circulation. Second reason is to commence flow in the riser tubes immediately after light up hot static head is kept as minimum as possible.

• Rapid heating & cooling (start up & shut down): As mentioned in Para 1, the controlled circulation boiler does not impose any thermal constraints on the drum and hence rapid cooling and heating of the boiler is possible. In NC boiler, rise in saturation temperatures is limited to maximum of 110OC/hr. Hence, the controlled circulation boiler can be started at a rate two to three times faster than NC boilers.

KORBA SIMULATOR

22

• Better cleaning of boiler: For effective acid washing, the acid has to be kept at certain temperature uniformly through the system. This is possible with the assistance of controlled circulation.

• Uniform expansion of pressure part and lower metal temperature:

This means lesser thermal stresses on the tubes. Because of controlled circulation, lower diameter tubes are used, which result in high mass flow rate thereby preventing departure from nucleate boiling (DNB) maintains a lower metal temperature.

USE OF RIFLE TUBES FOR FURNACE CONSTRUCTION This is one of the extraordinary features of 500 MW capacity boilers. Because of the excessive heat release in the burner zone of the furnace, the metal tubes constituting the furnace at that zone are exposed to the maximum temperature. This being a water-cooled furnace, the steam water mixture inside the tubes should effectively carry the heat from the burner zone of the furnace. In this zone, the tubes have an internally cut spiral like a rifle bore so that when water flows through the tubes, due to hot static heat, it takes a screwed path and attains a certain degree of spin by which the watness of the tube is always maintained. This prevents the tubes form departure from Nucleate boiling under all operating condition of the boiler and increases the circulation ratio. OVER FIRE AIR SYSTEM FOR NOX (OXIDES OF NITROGEN) CONTROL Industrial growth in the recent years has necessitated the need to have a cleaner and pollution free atmosphere, by controlling the production of industrial wastes with the application of improved technology. Power plants are the major sources of the industrial pollution by virture of the stanch emission in the atmosphere. These emissions contain mostly gases and dust particles, which have ill effect on the ecological system. In the 500 MW capacity boiler design, this aspect has been given due importance and certain technical improvements have been incorporated. These are tilting tangential firing and over fire air system. Tangential firing helps in keeping the temperature of the furnace low so that NOX emission is reduced considerably. In addition to the above the over fire air is provided which is used as combustion process adjustment technically for keeping the furnace temperature low and thereby low Nox formation. Each corner of the burner windbox is provided with two numbers of separate over fire air compartments, kept one above the other and the over fire air is admitted tangentially into the furnace.

KORBA SIMULATOR

23

The over fire air nozzles has got tilting arrangement and compartment flow control dampers for working in unison with the tilting tangential type burner system for effective control of Nox formation. AIR PRE-HEATER SYSTEM As compared to trisector air pre-heater in 200 MW units, 500 MW units have been incorporated with bisector air pre-heaters. This has been done for optimum utilisation of space and also improved system layout. This has resulted in the flexibility and efficient operation and maintenance of the air pre-heaters and the boiler as a whole. PRIMARY AIR SYSTEM The primary air system delivers air to the mills for coal drying and transportation of coal powder to furnace. The 500 MW units have two stage axial flow primary fans as compared to radial fans in 200 MW units. By introducing axial flow fans, the system efficiency has gone up as the axial flow fans consistently high efficiency at all operating loads. MILLING SYSTEM In the 500 MW units at SSTPS, Raymond’s Pressurised bowl mills have been installed. These are similar to the 200 MW mills except that 500 MW mills have vane wheel surrounding the bowl and external lubrication unit. Introduction of vane wheel has led to uniform distribution of primary air within the mill and less rejects. These mills are also supplied with weld overlay technology, which has increased the minimum wear life of grinding parts to 6000 hrs. I. D. FANS Unlike 200 MW units, the 500 MW units have been supplied with radial type I.D. fans. These fans have a lower speed and are less susceptible to wear and tear due to the abrasive flue gases. The control of the I.D. fans is achieved through a variable speed hydraulic coupling and motorised inlet damper. By introducing variable speed control through a hydraulic coupling the losses in the fan at various load has been minimised and efficiency of the fan has remained high at all operating conditions. ELECTROSTATIC PRECIPITATORS: Electro static precipitators are installed in the 500 MW units for minimising the particulate emission from the stack flue gases. There are four ESP passes for one unit of 500 MW and each is independently operated. The emitting electrodes are changed at high-ve voltage DC and the gas while crossing this charged path gets jonised. The ionised ash particles of the gas are attracted towards the collecting electrode, which is maintained at high +ve voltage. The ash collects at the collecting electrode and is periodically tapped to dislodge the accumulated ash. The ash falls into the hopper, which is evacuated by the ash handling system and taken out as slurry.

KORBA SIMULATOR

24

TECHNICAL SPECIFICATION OF 500 MW BOILER MAIN BOILER GENERAL SPECIFICATION

Manufacturer : M/s BHEL (C.E. Design)

Type : Balanced Draft, Dry bottom,

Single drum, Controlled

Circulation plus.

Type of Firing : Tilting Tangential

Minimum load at which the steam generator

can be operated continously with complete

flame.

: 2 Mills at 50%

Minimum load at which the steam generator can be

operated continuosly with complete flame.

Stability with oil support (% MCR)

: 20%

Maximum load for which individual mill

beyond which no oil support is required

: 50%

FURNACE SPECIFICATION

Wall : Water Steam cooled

Bottom : Dry

Tube arrangement : Membrane

Explosion/Implosion withstand capacity

(MWG) at 67% yields point.

: + 660

Residence time for fuel particles in the

furnace.

: 3 second

Effective volume used to calculate the

residence time (M3)

: 14770

Height from furnace bottom ash hopper to

furnace roof (M)

: 63.65

Depth (M) : 15.289

Width (M) : 18.049

Furnace projected area (M2) : 7610

KORBA SIMULATOR

25

Furnace volume (M3) : 14770

WATER WALLS

FRONT WALLS

Number : 283

OD (MM) : 51.00

Design thickness (MM) : 5.19

Pitch (MM) : 63.5

Actual thickness used (MM) : 5.6

Material : SA 210C

Total projected surface (M2) : 1160

Method of joining long tube : Butt weld

Total wt. of tubes (kgs) : 181000

Design pr. of tubes Kg/cm2 (ABS) : 207.3

Max. pressure of tubes Kg/cm2 (ABS) : 197.3

Design metal temp OC : 416

SIDEWALLS, REAR WALLS

& ROOF

Side walls Rear walls Roof

Number 444 283 142

OD (MM) 51 51 57

Design thickness (MM) 5.19 5.19 5.54

Pitch (MM) 63.5 63.5 127

Actual thickness used 5.6 5.6 5.7

Total projected surface area

of tubes (M2)

1430 930 220

Method of joining long tubes. BUTT WELD BUTT WELD BUTT WELD

Total wt. of tubes (Kgs) 277000 186000 45000

Design Pr. of tubes Kgs/cm2

(ABS)

207.3 207.3 204.9

Max pr. of tubes Kgs/cm2

(ABS)

193.3 197.3 192.3

Design metal Temp o C 417 417 416

KORBA SIMULATOR

26

WATER WALL HEADERS Lower drum WW outlet

No. of headers 1 5

Outside Dia (Dia (MM) 914 273

Design thickness (MM) 86 38.5

Actual thickness (MM) 89 45

Total wt. of headers (Kgs.) 166000 37300

Design pressure of headers kg/cm2 (ABS) 207.3 204.9

HEADERS Lower drum WW outlet

Max working pressure of headers Kg/cm2

(ABS)

197.3 192.4

Material specification SA-299 SA-106 Gr-B

DRUM

Material specification : SA-299

Design pressure Kg/cm2 (ABS) : 204.9

Design metal temp OC : 366

Max operating pressure Kg/cm2 (ABS) : 192.4

Actual thickness used for dished ends : 152.4

Overall length of Drum (MM) : 22070

OD of Drum (MM) : 2130

Internal dia (MM) : 1778

Corrosion allowance (MM) : 0.75

Number of distribution headers : 6

No. of Cyclone separator : 96

No. Of Secondary driers : 96

Shroud material : Carbon Steel

Max permissible temp differential between any

two parts of the drum (oC).

: 50

Water capacity at MCR conditions (in seconds) between

normal and lowest water level permitted

(up to LL trip)

: 10

Drum wt. with internals (tonnes) : 237.00

Drum wt. without internals (tonnes) : 215.00

KORBA SIMULATOR

27

BOILER WATER CIRCULATING PUMP

Number of pumps : (2 + 1)

CHARACTERISTICS

Type : Single suction double

discharge

Design Pressure : 207.55 Kg/cm2

(2965 lbf/in2)

Design temp : 366.2oC(691oF)

NORMAL OPERATING DUTY

Sunction Pressure : 193.27 Kg/cm2 (2761

1bf/in2)

Suction temp. : 348.9oC(660oF)

Specific gravity at pump Suction

at pumping temp.

: 0.5993

Qty. pumped : 47994.2 lit/min (12679 u.s

gal/min)

Differerential HEAD : 28.65 M (94.00 ft)

Differential Pressure : 1.708 Kg/cm2 (24.4

1bf/ in2)

Minimumm NPSH required above

Vapour Pressure

: 16.15 M (53 ft)

Pump efficiency : 84% Hot duty

BHP absorbed : 215 Hot duty-358 Cold

duty

KORBA SIMULATOR

28

MOTOR CHARACTERISITICS Hot duty Cold duty

Motor efficiency : 86% 88.6%

K.W. Input : 187 302

Power factor : 0.7 0.805

Overall efficiency : 72.2% 74.3%

Full load speed : 1450 rpm

Line current @ 6.6 KV : 23.3 amps 32.8 amps

Full load current : 36 amps

Motor starting current : 190 amps

Heat exchanger Hot duty

H.P. Inlet temp (max) : 55oC (130oF)

Allowable pr. drop : 0.7 kg/cm2 (10 1bf/in2)

Heat transfer - hot duty : 28980 kcal/hr. (115000 B.T.U./hr)

Heat transfer - cold duty : 30240 kcal/hr. (120000 B.T.U./hr)

H.P. cooling water flow : 200.62 lit/min (534.5 gal/min)

Weight (Approximate)

Pump case : 3541.2 kgs (7800 1bs)

Motor complete : 9534 kgs (21000 1bs)

Total weight : 13075.2 kgs (28800 1bs)

MOTOR CHARACTERISITICS

Type: : Wet Stator -Squirrel Cage

induction motor

Output : 400 H.P.

Service factor : 1.0

Winding : XLP

Motor case design temp. : 343.34OC (650OF)

KORBA SIMULATOR

29

BOILER WATER CIRCULATION PUMPS Each Boiler Water Circulation pump consists of a single stage centrifugal pump on a wet stator induction motor mounted within a common pressure vessel. The vessel consists of three main parts a pump casing, motor housing and motor covers. The motor is suspended beneath pump casing and is filled with boiler water at full system pressure. No seal exists between the pump and motor, but provision is made to thermally isolate the pump from the motor in the following respect:

• Thermal Conduction. To minimise heat conduction, a simple restriction in the form of thermal neck is provided.

• Hot Water Diffusion. To minimise diffusion of boiler water, a narrow annulus

surrounds the rotor shaft, between the hot and cold regions. A baffle ring restricts solids entering the annulus.

• Motor Cooling. The motor cavity is maintained at a low temperature by a heat

exchanger and a closed loop water circulation system, thus extracting the heat conducted form the pump.

• In addition, this water circulates through the stator and rotor bearings

extracting the heat generated in the windings and also provide bearing lubrication. An internal filter is incorporated in the circulation system.

• In emergency conditions, if low-pressure coolant to the heat exchanger fails, or

is inadequate to cope with heat flow from pump case, a cold purge can be applied to the bottom of the motor to limit the temperature rise.

Pump

The pump comprises a single suction and dual discharge branch casing. The case is welded into the boiler system pipework at the suction and discharge branches with the suction upper most. Within the pump cavity rotates a key driven, fully shrouded, mixed flow type impeller, mounted on the end of the extended motor shaft. Renewable wear rings are fitted to both the impeller and pump case. The impeller wear ring is the harder component to prevent galling. Motor

The motor is a squirrel cage, wet stator, induction motor, the stator, wound with a special watertight insulated cable. The phase joints and lead connections are also moulded in an insulated material. The motor is joined to the pump casing by a pressure tight flange joint and a motor cover completes the pressure tight shell.

KORBA SIMULATOR

30

•

KORBA SIMULATOR

31

The motor shell contains all the moving parts, except for the impeller. Below the impeller is situated an integral heat baffle which reduces the heat flow, a combination of convection and conduction, down the unit. A baffle wear ring-cum sleeve above the baffle forms a labyrinth with the underside of the impeller to limit sediment penetration into the motor. Should foreign matter manage to pass the labyrinth device into the motor enclosure, a filter located at the base of the cover end bearing housing strains it out. AUXILIARY COOLING CIRCUIT The motor is provided with its own auxiliary cooling circuit, which besides cooling the motor lubricates the bearings. The water is continuously circulated through the bearings, motor windings and the external heat exchanger, (cooler), by an auxiliary impeller (thrust disc) at the thrust bearing end of the motor shaft. When the motor is stationary, thermo-syphonic circulation takes place to remove conducted heat from the pump end of the motor. BEARINGS The motor rotor shaft is supported by water lubricated tilting pad type radial and thrust bearings mounted on the stator shell, thus making the motor internals into a separated construction independent of the motor pressure vessel. INTER FILTER A stainless steel woven wire strainer, fitted at the base of the reverse thrust plate, filters the liquid in the motor before it is circulated through the bearings after passing through the heat exchanger (cooler). The filter should be cleaned at normal maintenance periods, removing any accumulation of foreign matter in the motor cover. HEAT EXCHANGER A heat exchanger (cooler) is fitted to dissipate the heat generated by the motor winding. Brackets are provided on the motor case to mount the heat exchanger.High-pressure outlet and inlet-raised facings are situated bottom and top of the motor case respectively for connection to high-pressure heat exchanger/motor case stub pipes. Inter - connecting pipework is short and direct with the heat exchanger mounted as high as possible to promote good thermo-syphonic circulation when the unit is on hot standby.

KORBA SIMULATOR

32

PURGE AND FILL PIPING The purge and fill piping is used in association with boiler water circulation pump submerged motors. Depending on valve positions it can be used for filling or emptying the motor cavity of water, or for emergency purging of the cavity to prevent the ingress of hot boiler water should a leak occur in the cooling water system, or a gasket failure between pump and motor occur. Allowing high temperature boiler water to enter the cavity will damage the plastic insulation on the motor windings. During normal operation water is taken from the S.H. & R.H. spray water system then fed via a strainer and cooler before splitting three ways to service each circulation pump. If the pumps are to be filled when the S.H. spray water is out of service, a temporary connection can be made to take low pressure water from the reserve feed water tank. The valves, which service each circulation pump, can be opened and closed to make the system operate in out modes.

• Circulation pump filling. Water will flow through the filter then have its pressure and flow reduced through an orifice plate at the pump inlet. Drain lines down stream of the filter and the orifice will be closed.

• Circulation pumps emptying. The isolating valve upstream of the drain orifice

will be closed and water from the motor cavity will drain through open valves in the drain line downstream of the orifice.

• Piping blowdown. The isolating valve downstream of the filter will be closed and

drain line at the filter outlet open. Water will flow into the open drain.

• Circulation pumps purging. Water will flow as described in the filling mode but

the orifice bypass line will be opened to augment the flow. DRUM DESCRIPTION Connections at both ends to the chemical clean pipework, and at three points along its length to feed individual circulation pump suctions. Water will flow from the pumps through two discharge pipes into the front leg of the water wall inlet headers at the bottom of the furnace. Each discharge pipe is fitted with circulating pump discharge stop/check valves, which are controlled via sequence equipment to open and close as the pump is taken in and out of service. If, however all three pumps are out of service, all of the valves will open to enable thermosyphonic circulation to take place. Initiating any pump to restart will cause them all to close again then continue with the in and out of service regime. Controls for the pumps are located in the Contol room and comprise a SEQUENCE pushbutton, ammeter and a DUTY/STANDBY selector. Pump status is indicated on RUN/STOP lamps on the Firemen's Aisle Panel. The operating regime for the boiler water circulation pumps is two-duty/one stanby.

KORBA SIMULATOR

33

KORBA SIMULATOR

34

KORBA SIMULATOR

35

From the Waterwall inlet headers, water travels upwards through furnace wall tubing via furnace upper front rear and side headers into riser tubes, which direct a saturated steam/water mixture in to the steam drum. Furnace wall tubing is manufactured from a combination of both smooth and rifled bore tubing which permits the use of lower tube flow rates whilst still retaining full tube protection. The required distribution of water to give the correct flow rates through the various furnace wall circuits is achieved and maintained by the use of suitably sized orifices installed inside the water wall inlet headers at the inlet to each furnace wall tube. Orifice size varies for different circuits or groups of circuits depending on the circuit legth, arrangement and heat absorption. Perforated panel strainers are also located inside the water wall inlet headers to prevent the orifices blicking and to ensure an even distribution of water around the other inlet headers. The saturated steam/water mixture enters the steam drum on both sides behind a watertight inner plate baffle which directs the mixture around the inside surface of the durm to provide uniform heating of the drum shell. This eliminates thermal stresses from temperature differences through the thick wall of the drum, between the submerged and unsubmerged protions. Having travelled around this baffle the mixture enters two rows of steam enter the outer edge of the separator where it is separated from the steam. Nearly dried steam leaves the separators and passes through four rows of corrugated plate baskets where by low velocity surface contact the remaining moisture is removed.

SUPER HEATERS & REHEATERS SH LT

PENDENT

HORIZONTAL

STAGE-1

PANEL

STAGE-II

PLATEN

STAGE-III

Type Convection Radiant Radiant

Platen (Drainable/Non-

drainable)

--- Non- drainable Non-

drainable

Pendant

(Drainable/Non-

Drainable)

Drainable --- ---

Horizontal Headers

(Drainable/Non-

Drainable)

Drainable Drainable Drainable

Effective heating surface

area (M2)

12500 1660 1730

KORBA SIMULATOR

36

Gas flow path area (M2) 147 --- 286

Max steam side

Metal temp (oC)

408 509 575

Max gas side metal temp

(oC)

450 570 690

Type of flow (counter or

parallel)

Counter Parallel Parallel

Mat of tube support SS SS SS

OD (MM) 51.00 44.5 54.00

Total Number of tubes 708 444 408

TUBE PITCH (MM)

Parallel to gas flow 101.6 54.00 63.5

Across gas flow 152.5 254.00 762

Method of joining long

tube

<------------- Butt weld ------------------>

Total wt. of tubes (T) <------------ 1177 --------------------------->

REHEATERS

STAGE-1

RH RADIANT

WALL

STAGE-2

RH FINISHING PLATEN

RH INTER PENDENT

RH REAR PENDENT

Total heating surface (M2) 275 (proj.) 6200

Max operating pressure

(Kg/cm2)

47.68 47.00

Design pressure Kg/cm2

(ABS)

53.73 53.73

Max gas side metal temp oC 430 620

OD (MM) 63 63.5

Mean effecting length (perone

tube) MM (App)

17,500 27,000

Gross length (per one tube) MM

(App)

18300 38000

KORBA SIMULATOR

37

Total number of tubes 248 644

Total Wt. (Kgs.) 423300

Method of Joining long tube <----- Butt weld --------->

Headers

Max. Operating pressure

Kg/cm2 (ABS)

192.3 47.68

Design pressure (Kg/cm2)

(ABS)

204.9 53.73

Location (outside/inside gas

path)

Out side Out side

Total Wt. (Kgs.) 2111300 67000

SUPERHEATER AND REHEATER The arrangement, tube size and spacing of the Superheater and Reheater elements are shown on the attached “Schematic Flow Arrangement Diagram of Superheater and Reheater”. SUPERHEATERS The Superheater is composed of three basic stages of section; a Finishing Pendant Platen section, a Division Panel Section and a Low Temperature Section including LTSH, the Backpass Wall and Roof Sections. The finishing Section is located in the horizontal gas path above the furnace rear arch tubes and consists of assemblies spaced on 76.2 centres across the furnace width. The Division Panel Section is located in the furnace between the front wall and Pendant Platen Section. It consists of six front and six rear panel assemblies. The Low Temperature Sections and are located in the furnace rear Backpass above the Economiser Section. They are composed of assemblies spaced on centres across the furnace width. The Backpass wall and Roof Section forms the side front and rear walls and roof of the vertical gas pass. REHEATER The reheater is composed of 3 stages or sections, the Finishing Section the Front Platen Section and the Radiant Wall Section

KORBA SIMULATOR

38

The Finishing Section is located above the furnace arch between the furnace screen tubes and the Superheater Finish. It consists of assemblies. The Reheater Front and side Radiant Wall is composed of tangent tubes on centers across the furnace width. STEAM FLOW The course taken by steam from the steam drum to the superheater finishing outlet header can be followed on the attached illustrations, the “Schematic Flow & Arrangement of Superheater & Reheater”. The elements, which make up the flow path are essentially numbered consecutively. Where parallel paths exist, first one and then the other circuit are numbered. The main steam flow is: Steam drum - SH connecting tube - (1) -Radiant roof inlet header (2) - First pass roof front (3)- Rear (4) - Radiant tube outlet header (5) - SH SCW inlet header side (6) Backpasss side wall tubes (7) & (8) - Backpass bottom headers (9), (10) & (11) - Backpass Front, and rear (12) (21) - Backpass screen (13) Backpass roof (14)- Backpass SH & Eco. supports (15) SH & Eco. support headers (16) - LTSH support tubes (17) - SH Rear Roof tubes (18) - SHSC Rear wall tubes (19)- LTSH inlet header (22) - LTSH banks (23) (24) - LTSH outlet headers (25) - SH DESH link (26) - SH DESH (27) - Division panel (30) - Division Panel outlet header (31) - SH Pendent assembly (34) - SH outlet header (35). After passing through the high-pressure stages of the turbine, steam is returned to the reheater via the cold reheat lines. The reheater desuperheaters are located in the cold reheat lines. The reheat flow is. Reheater radiant wall inlet header (38) (39) - radiant wall tubes (40) (41) reheater assemblies (46) (47) - reheater outlet header (48) - Reheater load (49). After being reheated to the design temperature, the reheated steam is returned to the intermediate pressure section of the turbine via the hot reheat line. PROTECTION AND CONTROL As long as there is a fire in the furnace, adequate protection must be provided for the Superheater and Reheater elements. This is especially important during periods when there is no demand for steam, such as when starting up and when shutting down. During these periods of no steam flow through the turbine, adequate flow through the superhteater is assured by means of drains and vents in the headers, links and main steam piping. Reheater drains and vents provide means to boil off residual water in the reheater elements during initial firing of the boiler.

KORBA SIMULATOR

39

ARRANGEMENT OF SUPERHEATER AND REHEATER

KORBA SIMULATOR

40

KORBA SIMULATOR

41

Safety valves on the superheater main steam lines set below the low set drum safety valve, provide another means of protection by assuring adequate flow through the superheater if the steam demand should suddenly and unexpectedly drop Reheter safely valves, located on the hot and cold reheat piping serve to protect the reheater if steam flow through the reheater is suddenly interrupted. A power control valve on the superheater main steam line set below the low set superheater safety valve is provided as a working valve to given an initial indication of excessive steam pressure. This valve is equipped with a shut off valve to permit isolation for maintenance. The relieving capacity of the Power Control Valve is not included in the total relieving capacity of the safety valves required by the Boiler Code. During all start-ups, care must be taken not to overheat the superheater or reheater elements. The firing rate must be controlled to keep the furnace exit gas temperature from exceeding 5400C. A thermocouple probe normally located the upper furnace sidewall should be used to measure the furnace exit gas temperatures. NOTE:

• Gas temperature measurements will be accurate only if a shielded, aspirated

probe is used. If the probe consists of simple bare thermocouple, there will be an error, due to radiation, rustling in a low temperature indication. At 588OC actual gas temperature, the thermocouple reading will be approximately 10 degrees low. Unless very careful traverses are made to locate the point of maximum temperature, it is advisable to allow another 10 degrees tolerance, regardless what type of thermocouple probe is used.

• The 540OC gas temperature limitation is based on normal start up conditions,

when steam is admitted to the turbine at the minimum allowable pressure prescribed by the turbine manufacturer. Should turbine rolling be delayed and the steam pressure to permitted to build up the gas temperature limitation should be reduced to 510OC when the steam pressure exceeds two thirds of the design pressure before steam flow through the turbine is established.

Thermocouples are installed on various superheater and reheater terminals tubes, above the furnace roof, serve to give a continuous indication of element metal temperatures during start-ups (superheater) and when the unit is carrying load (Superheater and Reheater). In addition to the permanent thermocouples, on some units temporary thermocouples provide supplementary means of establishing temperature characteristics during initial operation. Steam temperature control for Superheater and Reheater outlet is provided by means of windbox nozzle tilts and desuperheaters. DESUPERHEATERS Super heater & Reheater temp Control

KORBA SIMULATOR

42

SUPER HEATER ATTEMPRATOR Type : Spray Stage : One Position in steam circuit : Between LT pendants

and SH panels.

Specification of material. : SA335 P12

Spray tube material : SA-213 T11

Super heater steam temp range that can be maintained from 54.43% to 100% of Boiler MCR.

: 540 oC

Max spray water flow rate and corresponding steam output (Kgs./hr.)

: 92,800 at 1566, 000 Kgs./hr.

Min spray water flow rate and corresponding steam out put Kg/hr. Reheat Emergency temp control attemperator

: 47,000 at 1550,000 Kg/hr.

REHEATER ATTEMPERATOR TYPE : SPRAY

No. of stages of attemperator : One

Position in the steam circuit : Before RH Radiant wal

Specification of the Material : SA-106 Gr-B

Spray nozzle Material : SA-213T & SS Tips

HEADERS

Length mm : 18,000

Design Pr. (Kg/Cm2) (abs) : 209.8

Max Working Pressure (Kg/Cm2)(abs) : 196

GENERAL Desuperheaters are provided in the superheater-connecting link and the reheater inlet leads to permit reduction of steam temperature when necessary and to maintain the temperatures at design values within the limits of the nozzle capacity. Temperature reduction is accomplished by spraying water into the path of the steam through a nozzle at the entering end of the desuperheater. The spray water comes from the boiler feedwater system. It is essential that the spray water be chemically pure and free of suspended and dissolved solids, containing only approved volatile organic treatment material, in order reheater and carry-over of solids to the turbine.

KORBA SIMULATOR

43

CAUTION: During start up of the unit, if desuperheating is used to match the outlet steam temperature to the turbine metal temperatures, care must be exercised so as not to spray down below a minimum of 100 C above the saturation temperature at the existing operating pressure. Desuperheating spray is not particularly effective at the low steam flows of start up. Spray water may not be completely evaporated but be carried through the heat absorbing sections to the turbine where it can be the source of considerable damage. During start up alternate methods of steam temperature control should be considered. The location of the desuperheaters helps to ensure against water carry - over to the turbine. It also eliminates the necessity for high temperature resisting materials in the desuperheater construction. SUPERHEATER DESUPERHEATERS Two spray desuperheaters are installed in the connecting link between the superheater low temperature pendant outlet header and the superheater division panel inlet headers. REHEATER DESUPERHEATERS Two spray type desuperheaters are installed in the reheater inlet lead near the reheater radiant wall front inlet header. ECONOMISER Type : Non Steaming

Water side effecting heating surface area (M2) : 7810

Gas side effecting heating Surface area (M2) : 10210

Gas flow path area (M2) : 128

Design pressure of tubes Kg/cm2 : 209.8

OD of Tubes (MM) : 51.00

Actual thickness tubes (MM) : 5.6

Length of Tubes (MM) (App) : 2,15,000

Pitch (MM) : 101.6

Total Wt. of Tubes (Kgs.) : 4,95,00

KORBA SIMULATOR

44

BARE TUBE ECONOMISER The function of the economiser is to preheat the boiler feedwater before it is introduced into the Steam drum by recovering some of the heat of the flue gas leaving the boiler. Refer the " Schematic Flow and Arrangement Diagram of Water & Saturated Steam Circuits". The economiser is located in the boiler backpass. It is composed of two banks of 156 parallel tube elemets (3) arragned in horizontal rows in such a manner that each row is in line with the row above and below. All tube circuits originate from the inlet header (2) and terminate at oulet headers (4) which are connected with the economiser outlet header (7) through three rows of hanger tubes (6). Feedwater is supplied to the economiser inlet head (1) (2) via feed stop and check valves. The feedwater flow is upward through the economiser, that is, counterflow to the hot flue gases. Most efficient heat transfer is, thereby, accomplished, while the possibility of steam generation within the economiser is minimised by the upward water flow. From the outlet header the feedwater is lead to the steam drum through the economiser outlet links (5) (6). The economiser recirculating lines, which connects the economiser inlet lead header (2) with the furnace lower rear drum (14), provide a means of ensuring a water flow through the economiser during startups. This helps prevent steaming. The valves in these lines must be open during unit startup until continuous feed water flow is established. WATER COOLED FURNACE WELDED WALL CONSTRUCTION The furnace walls are composd of 51.0D. Tubes on 63.5" centers. The space between the tubes is fusion welded to from a complete gas tight seal. Some of the tube ends are swaged to a smaller diameter while other tubes are bifurcated where they are welded to the outlet headers and lower drum nipples. The furnace arch is composed of 63.5 O.D. fusion welded tubes, 76.2 (typical) centers. The backpass walls and roof are composed of 63.5 O.D. fin welded tubes on 154.4 centers. The furnace extended sidewalls is composed on O.D. fin welded tubes on 127 centers.

KORBA SIMULATOR

45

KORBA SIMULATOR

46

KORBA SIMULATOR

47

The backpas front (furnace) roof is composed of 51.0. O.D. tubes, peg fin welded on 152.4 centers. The backpass rear roof is composed of 51.0 O.D tubes peg fin welded on 152.4 centers. All peg-finned tubes are normally backed with a plastic refractory and skin casing, which is seal, welded to form a gas tight envelope. Where tubes are spread out to permit passage of superheater elemets, hanger tubes, observation ports, soot blowers, etc., the spaces between the tubes and openings are closed with fin material so a completely metallic surface is exposed to the hot furnace gases. Poured insulation is used at each horizontal buckstay to form a continous band around the furnace thereby preventing flue action of gases between the casing and water walls. BOTTOM CONSTRUCTION Bottom designs used in these coal-fired units are of the open hopper type, often referred to as the dry bottom typ. In this type of bottom construction two furnace water walls, the front and rear walls, slope down toward the centre of the furnace to form the inclined sides of the bottom. Ash and/or slag from the furnace is discharged through the bottom opening into n ash hopper directly below it. A seal is used between the furnace and hopper to prevent ambient air being drawn into the furnace and disturbng combustion fuel/air rations. The seal is affected by dipping seal plates, which are attached around the bottom opening of boiler furnace, into a water trough around the top of the ash hopper. The depth of the trough and seal plates will accommodate maximum downward expansion of the boiler (predicated (320.3 mms). Feedwater enter the unit through the economizer elements (1) (2) (3) (4) (5) (6) and is mixed with boiler water in the steam drum (7). Water flows from the drum (7) through the downcomers (8) to the pump suction manofld (9). The boiler-circulating pump (10) takes water from the suction manifold and discharges it, via the pump discharge lines (12), into the furnace lower front inlet header (13). Furnace lower water wall right and left side headers (15) assure proper distribution to the rear heater (14). In the waterwall inlet headers the boiler water passes through strainers and then through orifices, which feed the furnace wall tubes, the economiser recirculating lines. The water rises through furnace wall tubes where it absorbs heat. The front wall tubes (16), rear tubes (17), rear wall hanger tubes (19), rear arch tubes (18), rear screen tubes (21), extended sidewall tubes (2) and sidewall tubes (22) from parallel flow paths. The resulting mixture of water and steam collects in the waterwall outlet headers (23) (24) (25) (26) and is discharged into the steam drum (7) through the riser tubes (27). In

KORBA SIMULATOR

48

the steam drum the steam and water are separated (see "Drun Internals"), the steam goes to the superheater (see "Supergeater and Reheater") and the water is reurned to the waterside of the steam drum to be recirculated. WATERWALL INLET HEADERS The waterwall inlet headers are rectangular ring shaped manifold at the bottom of the furnace. Downcoming pipes enter into the furnace lower front inlet header. Furnace lower waterwall right and left side headers assure proper distribution to the rear heads. In the waterwall inlet headers the boiler water passes through screen and then through orifices, which feed the furnace wall tubes. The screen consists of a number of panels with 2/16" perforations. The panels are secured in the inlet drums with clamps. The panels are made in sections to facilitate removal and replacement. Each orifice is installed on the orifice mount adapter tack welded to the drum interior wall. A marman clamp holds the orifice on the orifice mount. NOTES:

1. Initial boiling out and acid cleaning operation to be completed before installing orifices.

2. Screens however must be installed

3. Orifice and screen assemblies retained on subsequent acid cleaning operation

and removed for inspection purposes only. H.P. CHEMICAL DOSING SYSTEM Intermittent H.P. Chemical dosing is used to inject Tri-sodium Phosphate (T.S.P.) into the boiler water so that a phosphate reserve is maintained. T.S.P will precipitate any undesirable hardness salts contained in the water into a form of free flowing sludge, which can be removed by blowdown. A solution of T.S.P. will be made ready in the mixing tank using a motor operated stirrer and make up water as necessary. When prepared, the solution will be transferred by gravity feed to the metering tank ready for injection into the boiler steam drum in quantities determined by chemical analysis.

KORBA SIMULATOR

49

The level of solution in the tanks can be observed through side mounted gauge glasses. Further monitoring is provided by level switches which initiate an alarm when the level in the metering tank is high / low. Drains from the gauge glasses and tank overflows empty into an open drains system. Solution is pumped from the metering tank by one of the two 100 % duty H.P. dosing pumps (one standby) into the steam drum. Both pump system are indentical and include a suction filter and a discharge pressure relie valve. Each relief valve discharges into the open drain system. FUEL FIRING SYSTEM INTRODUCTION The information Contained in this chapter relates to the fuel (oil & coal) system and fuel / combustion equipment under supply of BHEL for 500 MW boilers. FUEL OIL SYSTEM The fuel oil system prepares any of the two designated fuel oil for use in oil burners (16 per boiler, 4 per elevation) to establish initial boiler light up of the main fuel (pulverised coal) and for sustaining boiler low load requirements upto 15 % MCR load. To achieve this, the system incorporates fuel oil pumps, oil heaters, filters, steam tracing lines which together ensure that the fuel oil is progressively filtered, raised in temperature, raised in pressure and delivered to the oil burners at the requisite atomising viscosity for optimum combustion efficiency in the furnace. COAL SYSTEM The coal system prepares the main fuel (pulverised coal) for main boiler furnace firing. To achieve this the raw coal from overhead hopper is fed through pressurised coal valve, SECOAL nuclear monitor, and gravimetric feeder and into mills where it is crushed and reduced to a pulverised state for optimum combustion efficiency. The pulverised coal is mixed with a primary airflow, which carries the coal air mixture to each of 4 corners of the furnace burner nozzles and into furnace. BURNER NOZZLES Both the oil and coal burner nozzles fire at a tangent to an imaginary circle at the furnace centre. The turbulent swirling action this produces, promotes the necessary mixing of the fuels and air to ensure complete combustion of the fuel. A vertical tilt facility of the burner nozzles, which is controlled by the automatic control system of boiler, ensures a constituent reheat outlet steam temperature at varying boiler loads.

KORBA SIMULATOR

50

TILTING TANGENTIAL FIRING SYSTEM GENERAL In the tangential firing system the furnace itself constitutes the burner. Fuel and air introduced to the furnace through four windbox assemblies located in the furnace corners. The fuel and air streams from the windbox nozzles are dissected to a firing circle in the centre of the furnace. The rotative or cyclonic action that is characteristic of this type of firing is most effective in turbulently mixing the burning fuel in a constantly changing air and gas atmosphere AIR AND FUEL NOZZLE TILTS The air and fuel stream are vertically adjustable by means of movable air deflectors and nozzle tips, which can be tilted upward or downward through a total of approx. 60 degrees. This movement is effected through connecting rods and tilting mechanism in each windbox compartment, all of which are connected to a drive unit at each corner operated by automatic control. Provision is given in UCB to know the position of nozzle tips during operation. The tilt drive units in all four corners operate in unison so that all nozzles have identical tilt positions. WINDBOX ASSEMBLY The fuel firing equipment consists of four windbox assemblies located in the furnace corners. Each windbox assembly is divided in its height into number of sections or compartments. The coal compartments (fuel air compartment) contain air (intermediate air compartments). Combustion air (secondary air) is admitted to the intermediate air compartment and each fuel compartment (around the fuel nozzle) through sets of louvre dampers. Each set of dampers is operated by a damper drive cylinders located at the side of the windbox. The drive cylinders at each elevation are operated either remote manually or automatically by the Secondary Air Damper Control System in conjunction with the Furnace Safeguard Supervisory System. Some of the (auxiliary) intermediate air compartments between coal nozzles contains oil gun. (Refer contract assembly drawing for details).Retractable High Energy Arc (HEA) ignitors are located adjacent to the retractable oil guns. These ignitors directly light up the oil guns. Optical flames scanners are installed in flame scanner guide pipe assemblies in the auxiliary are compartments. The scanners sense the ultraviolet (UV) radiation given off by the flame and thereby prove the flame. They are used by Furnace safeguard Supervisory System to initiate a master fuel top upon detection of flame failure in the furnace.

KORBA SIMULATOR

51

KORBA SIMULATOR

52

AIR FLOW CONTROL AND DISTRIBUTION Total airflow control is accomplished by regulating fan dampers or fan speed. Air distribution is accomplished by means of the individual compartment dampers. The airflow to the air boxes can be equalised by observing and equalising the reading of the flowmeters located in the hot air duct to windbox. TOTAL AIR FLOW In order to ensure safe light-off conditions, the pre-optional purge airflow (at least 30 % of full load volumetric air flow) is maintained during the entire warm-up period until the unit is on the line and the unit load has reached the point where the airflow must be increased to accommodate further load increase. To provide proper air distribution for purging and suitable air velocities for lighting off, all auxiliary air dampers should be open during the purge period, the lighting off and the warm-up period. After the unit is on the line, the total required amount of air (total air flow) is a function of the unit load. Proper airflow at a given load depends on the characteristics of the fuel fired and the amount of excess air required (see note) to satisfactorily burn the fuel. Excess air can be determined through flue gas analysis (Orsat measurements). The optimum excess air is normally defined as the O2 at the economiser outlet that

produces the minimum opacity. Operation below the optimum excess air will result in high opacity due to unburned carbon where as operation above the optimum excess air will result in high capacity due to excessive H2 SO4 condensation. Operation below

recommended range will result in excessive black smoke and operation above this range will result in excessive white smoke. NOTE: The most suitable amount of excess air for a particular unit, at a given load and with a given fuel must be determined by experience. This is best done form observation of furnace slagging conditions. Slagging tendency of a particular fuel may dicatate an increase of operating excess air. AIR FLOW DISTRIBUTION The function of the windbox compartment dampers is to proportion the amount of secondary air admitted to an elevation of fuel compartments in relationship to that admitted to adjacent elevation of auxiliary air compartments. Windbox compartment damper positioning affects the air distribution as follows: Opening up the fuel - air dampers or closing down the auxiliary air dampers increases

KORBA SIMULATOR

53

the air flow around the fuel nozzle Closing down the fuel air dampers and opening the auxiliary air dampers decrease the air flow directly around the fuel stream. Proper distribution of secondary air is important for furnace stability when lighting off individual fuel nozzle, when firing at low rates and for achieving optimum combustion condition in the furnace at all loads. Proper distribution of secondary air also has an effect on the emission of pollutants form coal fired units. As the unit increases the quantity of Nitrogen Oxide (NO) Produces in a furnace (due to the oxidation of nitrogen in the fuel) increases and the upper elevations of fuel nozzles are placed in service. The quantity of NO produced can be reduced by limiting the amount of air admitted to the furnace adjacent to the fuel and increasing the quantity of air admitted above the fire (over fire air). When the unit has reached a predetermined load (app. 50 %) the over-fire air dampers should open and modulate as a function of unit load until, at maximum continuous rating (MCR) when upto 15 % of the total air is admitted to the furnace as over fire air. The optimum ratio of over fire air to fuel and auxiliary air, as well as the optimum tilt position of the over-fire air nozzles, to produce a minimum NO emission consitent with satisfactory furnace performance must be determined through flue gas testing (i.e. measurement of NO) during initial operation of the unit. The correct proportion of air between fuel compartment and auxiliary air compartments depends primarily on the burning characteristics of the fuel. It influences the degree of mixing, the rapidity of combustion and the flame pattern within the furnace. The optimum distribution of air for each individual installation and for the fuel used must be determined by experience. The wind-box compartments are normally provided with drive (except end air compartments) so they may be operated by a secondary air damper and over-fire air control system in conjuction with the furnace safeguard supervisory system. When on automatic controls the system should provide modulation of the auxiliary air dampers as required to maintain a pre-set windbox-to-furnace differential pressure. The fuel air dampers should be closed prior to and during light off. When the fuel elevation is proven in service, the associated fuel-air dampers should open and be positioned in proportion to the elevation-firing rate. Normally the end air compartments are [provided with manual adjustment, which can be kept in the required position during commissioning of the unit. FUEL OIL FIRING SYSTEM FUELS A coal-fired unit incorporates oil burners also to minimum oil firing capacity of 15 % of boiler load for the reason of,

1. To provide necessary ignition energy to light-off coal burner

2. To stabilise the coal flame at low boiler/burner loads

3. As a safe start-up fuel and for controlled heat input during light-off.

KORBA SIMULATOR

54

Auxiliary steam is utilised in boiler for following purposes:

• For atomising the HFO at the oil gun.

• For tank heating, main heating and heat tracing of HFO.

• To preheat the combustion air at the steam coil air heater and to warm up the main air heater (this reduces Sulphur-oxide condensation and thus cold end corrosion of main air heater)

With above provisions and with proper oil, steam and combustion air parameters at the burner, HFO is safely fired in a cold furnace BURNER ARRANGEMENT In a tangentially fired boiler, four tall windboxes (combustion air boxes) are arranged, one at each corner of the furnace. The coal burners or coal nozzles are located at different levels on elevations of the windboxes. The number of coal nozzle elevations are equivalent to the number of coal mills. The same elevation of coal nozzle at 4 corners is fed from a single coal mill. The coal nozzles are sandwitched between air nozzles or air compartments. That is, air nozzles are arranged between coal nozzles, one below the bottom coal nozzle and one above the top coal nozzle. If there are ‘n’ numbers of coal nozzles per corner there will be (n + 1) numbers of air nozzle per corner. The coal fuel and combustion air streams from these nozzles or compartments are directed tangential to an imaginary circle at the centre of the furnace. This creates a turbulent vortex motion of the fuel air and hot gases which promotes mixing ignition energy availability, combustion rate and combustion efficiency. The coal and air nozzles are tiltable ± 30 0 about horizontal, in unison, at all elevations and corners. This shifts the flame zone across the furnace height for the purpose of steam temperature control. The air nozzle in between coal nozzles is termed as Auxiliary Air Nozzles, and the top most and bottom most air nozzles as END Air Nozzles. The coal nozzle elevation are designated as A,B, C,D etc., from bottom to top, the bottom end air nozzle as AA and the top end air nozzles as XX. The auxiliary air nozzles are designated by the adjacent coal nozzles, like AB, BC, CD, DE ....... etc. The four furnace corners are designated as 1,2,3, and 4 in clockwise direction looking from top, and counting front water wall left corner as “1”.

KORBA SIMULATOR

55

Each pair of coal nozzle elevation is served by one elevation of oil burners located in between. For example in a boiler with 8 mills or 8 elevations of coal nozzles, there are 16 oil guns arranged in 4 elevations, at auxiliary air nozzles AB, CD, EF, & GH. Heavy fuel oil can be fired at the oil guns of all elevations. Each oil gun is associated with a high-energy arc ignitor. COMBUSTION AIR DISTRIBUTION Of the total combustion air, a portion is supplied by primary air fans, which go to coal mill for drying and pulverising the coal and carrying it to the coal nozzles. The Primary Air flow quantity is decided by coal mill load and the number of coal mills in service. The primary airflow rate is controlled at the air inlet to the individual mills by dampers. The balance of the combustion air, referred as ‘secondary air’ is provided by FD fans. A portion of secondary air (normally 30% to 40 %) called ‘Fuel Air’ is admitted immediately around the coal fuel nozzles (annular space around the casting insert) into the furnace. The rest of the secondary air called ‘Auxiliary Air’ is admitted through the auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air + auxiliary air) is dictated by boiler load and controlled by FD fan blade pitch. The proportioning of air flow between the various coal fuel nozzles and auxiliary air nozzles is done based on boiler load, individual burner load, and the coal / oil burners in service, by a series of air dampers. Each of the coal fuel nozzles and auxiliary and end air nozzles is provided with a knock-knee type regulating dampers, at the air entry to individual nozzle or compartment. On a unit with 8 mills there will be 8 fuel air dampers, 7 auxiliary air dampers, 2 end air dampers and 2 over fire air dampers per corner. Each damper is driven by an air cylinder positioned set, which receives signal from ‘Secondary Air Control System’. The dampers regulate on elevation basis, in unison, at all corners. FURNACE PURGE Traces of unburnt fuel air mixture might have been left behind inside the furnace of some fuel or might have entered the furnace through passing valves during shutdown of the boiler. Lighting up a furnace with such fuel air accumulation leads to high rate of combustion, furnace pressurisation and to explosions at the worst. This is avoided by the ‘Furnace Purge’ operation during which 30 % of total air flow is maintained for above 6 minutes to clear off such fuel accumulations and fill the furnace with clean air, before lighting up.

KORBA SIMULATOR

56

During furnace purge, all the elevations of auxiliary and end air dampers are opened to have uniform and through purging across the furnace volume. BOILER LOW LOAD OPERATION During initial operations upto about 30 % boilers loading (and also during furnace purge) all the auxiliary and end air dampers modulate to maintain a predetermined (approx. 40 mm WC) set point differential pressure between the windbox and furnace. During this period also, 30-40 % of total airflow is maintained to have an air rich furnace and to avoid possible unhealthy furnace conditions. Again all the auxiliary and end air dampers are open to distribute the excessively admitted air away from the operating burners and to pass only the necessary air behind the operating burners at appropriate velocity, for successful burner light up and stable flames. Around 40 mm of windbox of furnace differential is the pressure estimated as required to admit 30-40 % of airflow with the entire auxiliary and end air dampers modulating with reasonable opening. Whenever one or more oil burners are put into service the associated elevation of auxiliary air dampers modulate as a function of oil header pressure, to provide required combustion air. The other auxiliary air dampers continue to maintain 40 mm windbox to furnace differential. At boiler load less than 30 % MCR, each elevation of oil burners shall not be loaded more than 10 % MCR (if high capacity provided), since no adequate combustion air will be available behind oil burners, under this operating conditions. If found necessary total airflow may be marginally increased for better flame conditions. BOILER LOAD ABOVE 30 % When the unit load exceeds 30 % MCR, the differential set point is changed to a higher setting (approx. 100 mm WC). Simultaneously, the auxiliary air dampers associated with the coal or oil elevations not in service close in timed sequence starting with the upper elevations of dampers and progressing to the lowest elevation. The above 100 mm WC differential is the predicted value required to admit the total secondary air at design air velocities with all dampers opened to reasonable percentage. When the unit load is reduced below 30 % loading, the auxiliary air dampers open in a timed sequence starting with the lowest elevation of dampers. Simultaneously, the differential set point change to its lower setting.

KORBA SIMULATOR

57

The auxiliary air dampers associated with the oil elevations modulate as a function of oil header pressure when oil is being fired and opens more and more with increased firing rate. Otherwise, they open and close with balance of the auxiliary air dampers. The bottom end air damper is normally kept open to a fixed predetermined position to reduce unburnt coal dust fall - out. All the auxiliary air dampers maintain the status quo upon a boiler trip and will open fully when both FD fans are off. FUEL AIR DAMPERS Its operation is independent of boiler load. All fuel air dampers are normally closed. They open fifty seconds after the associated feeder is started and a particular speed reached. It modulates as function of feeder speed. Fifty seconds after the feeder is removed from service, the associated fuel air dampers close. The fuel air dampers will open fully when both FD fans are off. FUEL OIL ATOMISATION Atomisation is the process of spraying the fuel oil into fine mist, for better mixing of the fuel with the combustion air. While passing through the spray nozzles of the oil gun, the pressure energy of the atomising steam breaks up the oil stream into fine particles. Poorly atomised furl oil would mean bigger spray particles. Which takes longer burning time, results in carryovers and makes the flame unstable due to low rate of heat liberation and incomplete combustion. Viscosity of the oil is another major parameter, which decides the atomisation, level. For satisfactory atomisation the viscosity shall be less than 28 centistokes. External mix type steam atomised oil guns suitable for both LFO and HFO have been provided. Atomisers of this type are widely known as J-tips. The atomiser assembly consist of nozzle body welded on to the gun body, back plate, spray plate and cap nut. HEAVY FUEL OIL RECIRCULATION The HFO heater sets are located at a considerable distance from the boiler-burner proper.Before putting in the first burner into service, it is necessary to warmup the long oil supply lines from the heater to the burners, so that the oil does not get cooled in the pipings and that the oil at correct atomising temperature is available at the

KORBA SIMULATOR

58

burners. To achieve this the heater oil is circulated upto the burners and back to the oil tank through HFO return lines, till adequate temperature is reached near the burners. For the above purpose there are two HFO recirculation loops. One is called the long Recirculation, which is through the main trip valve, supply flow meter and flow control valve in the HFO supply line, supply ring header and HFO return lines. Long recirculation is the effective one, which circulates oil right upto the burner valve inlet, nears the corner risers. Long recirculation is not possible if no control power is available and during mater fuel trip. During such occasions the other partial recirculation loop called short recirculation is employed. This later loop bypasses the boiler area piping and connects the HFO return line to the HFO supply line before the HFO main trip valve and supply flow meter, short recirculation valve is opened when the main trip valve is closed, essentially for warming up the main lines. Before opening the main trip valve or the first burner trip valve, the short recirculation valve is closed. A HFO return trip valve (HORV) is installed in the long recirculation loop. With this valve open, large volumes of HFO can be circulated upto the burners and initial warming up of the pipings can be faster. When one or more burners are firing (i.e. when HORV is shut) still a small amount of hot oil is constantly recirculated through a restricting orifice arranged across HORV. This constant recirculation keeps the HFO return line always warm, prevents solidification of oil at dead ends and ensures uniform temperature in the piping. This orifice is sized for a circulation flow rate of about 7 - 10 % of maximum oil firing rate. During initial commissioning, this recirculation flow rate shall be checked and if found necessary orifice size be suitably changed or the regulating valve opening be adjusted. The HFO return flow meter is installed across the HORV in series with the constant recirculation orifice, rather than in the common return line, for better rangeability. The flow metering should be accounted only when HORV is closed. When the boiler is firing on coal or no oil burner is fired it is recommended to open Heavy Oil Main Trip Valve and Return Valve to circulate the oil continuously. This will enable the operator to cut in the oil gun immediately when required. The amount of oil circulation however is to be restricted to avoid shooting up of tank temperatures and hence the flow control valve may be throttled to reduce the return oil flow rate. OIL FLOW CONTROL This is remote manually done by varying oil flow control valve opening. The need for varying the oil burner load and the normally adopted practice is described in the following lines.

KORBA SIMULATOR

59

SYSTEM REQUIREMENT The maximum total output of oil burners is 30 % of the boiler MCR. This meets the turbine synchronisation needs before firing coal burners. Each oil burners capacity is about 2 % of boiler MCR. For coal burner ignition and coal flame stabilisation a minimum oil burner output, equivalent to 10-20 % of maximum coal burner capacity is required. This roughly corresponds to 40 to 50 % rating of an oil burner. For the exact capacities refer to performance data sheet (oil burners and ignitors) The oil burner output is a function of oil pressure at the oil gun and the normal turndown range of the oil burner is 3: 1. For steam atomised oil burner, the oil pressure at the oil gun shall not fall below 2.5 kg/sq cm2(g) to ensure good atomisation and stable flames. The oil burners have to be opted at loads, lower than the maximum rating for reasons mentioned below.

1. During cold start-ups of the boiler, to have a controlled and gradually increasing heat loading, to avoid temperature stresses on pressure part materials, as dictated by boiler start up curves.

2. To conserve fuel oil by operating the oil burners just at the “Coal flame

stabilisation” requirements. Oil Flow Control Valve and Minimum Pressure Control Valve Function The oil header pressure is maintained constant at all loads, at the upstream of oil flow control valve by a relieving type backpressure control valve installed after the pump. The flow control valve essentially does the function of regulating the boiler fuel oil firing rate. The valve opening can be varied from the remote depending upon the no of burners firing and the firing rate. The minimum pressure control valve ensures a smooth starting up boiler. To start with, the Heavy Fuel Oil Heater Trip Valve and HORV are opened. Once the temperature at the boiler front is adequate, the heavy fuel oil flow control valve is kept at the predetermined minimum firing opening to restrict the firing rate. This can be done by setting the required header pressure and maintaining the same through the pneumatic pressure controller. The burner trip valves are then opened and burners are put into service. The burners are operated only by pair mode.

KORBA SIMULATOR

60

KORBA SIMULATOR

61

When more no of burners are brought into service the heavy fuel oil header pressure will experience a sudden dip. The header pressure will be automatically maintained by the pressure control loop in the flow control valve. If this pressure control loop is not in service, it is always a good operator’s practice to increase the header pressure before additional burners are brought into service. The position transmitter or position limit switches mounted on the flow control valve serve to indicate the status of opening of control valve. An UCB display of control valve outlet pressure and the number of burners in service are the correct guidance for the operator. The fuel oil flow meter reading at panel could also be of equal assistance. OIL FIRING HEAVY FUEL OIL BURNERS Type

Burner : Tilting Tangential, corner fired

Oil gun : Parallel pipe, auto retractable

Atomiser : External mix, constant pressure, Steam

atomised

Air Nozzle : Suare to round

Atomiser designation : J 18

Atomiser spray angle : 90o

fuel : Fuel oil to IS:1593, 1971, Grade LV-MV-

HV

Design capacity : 7.5% MCH heat imput/4 guns

Number off : 16.4 per elevation

Location : Elevation AB, CD, EF & GH

Oil Firing rate maximum : 2250 Kg/hr/gun

Oil Firing rate minimum : 750 Kg/hr/gun

Turn Down : 3 to 1

Oil pressure at maximum rating

pressure

: 13 Kg/cm2 (g) at gun

Oil viscosity : 15-28 CST at gun

Atomising steam flow at Maximum

rating.

: 160 Kg/hr/gun

KORBA SIMULATOR

62

Atomising steam flow at minimum

rating

: 215 Kg/hr/gun

Atomising steam pressure : 5.25 Kg/cm2 (q) at the gun; constant at

all loads

Minimum allowable atomising steam

pressure

: 4.5 kg/cm2 (g) at the gun

Atomising steam quality : 20 to 30oC superheated

FUEL OIL PUMP

Make : BORNEMAN

Type : Screw

no. off : 3 (1 standbvy)

Rotation : Clockwise from Motor end

Capacity : 800 lpm at 150 cst.

Pressure : 30 Kg/cm2

Speed : 1450 rpm

Pump KW : 83 KW

Motor KW : 90 KW

Suction Filter Mesh : 500 Micron

Discharge Filter Mesh : 250 Micron

IGNITORS

Type : High Energy Arc Ignitor

Output 4 sparks/sec; : 12 Joules/spark

Sparking Time : 10 seconds

Rating : 110 V AC 50 Hz.

High Energy Arc type electrical ignitors are provided which can directly ignite the heavy fuel oil. The main features of this system are

• An exciter unit which stores up the electrical energy and releases the energy at a high voltage and short duration.

• A spark rod tip which is designed to convert the electrical energy into an

intensive spark.

KORBA SIMULATOR

63

• A pneumatically operated retract mechanism which is used to position the

spark rod in the firing position and retract to the non-firing position.

Each descrete spark provides a large burst of ignition energy as the current reaches a peak value of the order of 2000 amps. These sparks are effective in lighting of well-

atomised oil spray and also capable of blasting off any coke particle or oil muck on the surface of the spark rod.

For a reliable ignition of oil spray by the HEA ignitor, it is very much necessary to

maintain the following conditions:

1. The atomisation is maintained at an optimum level. All the atomising parameters such as oil temperature, steam pressure, clean oil gun tips etc., are maintained without fail. The atomising steam shall be with 20-degree superheat minimum.

2. The cold legs are minimum. the burner fittings are well traced and insulated.

3. The spark rod tip is located correctly at the optimum location.

4. The oil gun location with respect to the diffuser and the diffuser location with

respect to the air nozzle are maintained properly.

5. The control system is properly tuned with ignitor operation. The time of commencing of all the operational sequences is properly matched.

6. It may become necessary to close the air behind the ignitors, during the light off

period for reliable ignition. This must be established during the commissioning of the equipment and proper sequence must be followed.

The following facts must be born in mind to understand the ignitors and the system clearly :

• The spark rod life will be drastically reduced if left for long duration in the advanced condition when the furnace is hot.

• Too much retraction of spark rod inside the guide tube will interfere with nozzle

tilts and may spoil the guide tube.

• A minimum discharge of 300 kg/hr of oil is essential for a reliable ignition.

• A plugged oil gun tip may result in an unsuccessful start.

KORBA SIMULATOR

64

• A cold oil gun and hoses cause quenching of oil temperatures and may lead to an unsuccessful start. In such cases warming up by Scavenging prior to start is necessary.

FUEL OIL GUN ADVANCE / RETRACT MECHANISM The atomiser assembly of an operating oil gun is protected for the hot furnace radiation by the flow of fuel out / steam which keeps it relatively cool. Once the burner is stopped there is no further flow of oil/steam. Under such situation it is required to withdraw the gun from firing position to save it from possible damage due to over heating. In the system provided, the oil gun is auto advance, auto retractable. It is diven by a pneumatic cylinder and a 4 way dual coil solenoid pilot control valve, with a stroke length of 330 mm. There are three position limit switches, one for, “gun engaged” position, another for “Gun advanced” and the third for “Gun retracted” position, which have been suitably interlocked into furnace safeguard supervisory. system logic’s for safe and sequenced operation. STEAM SCAVENGING OF FUEL OIL GUNS Before stopping the oil burner, the oil gun is scavenged with steam to keep the small intricate passages of the atomiser parts clean.

• In the autoprogrammed burner stop sequence, a planned shut down is followed by steam scavenging the oil side for quite sometime, to achieve this requirement.

• During emergency tripping of the burners or boiler the oil gun is neither

scavenged nor retracted automatically. Normally such emergency trip may last only for a shot while and the fuel oil guns shall be re-started or local manually scavenged immediately on resuming boiler operation.

BURNER NOZZLE VALVES The burner nozzle valves are of pneumatic diaphragm type. The oil valves are provided with facility to adjust the opening time. The opening is slow to avoid a pressure dip in the oil header. The closing of the valves are instantaneous. It is very important to check these valve periodically for any seat leakage. SYSTEM VENTS AND DRAINS Fuel oil heaters strainers and lines are provided with ventcocks or valves on oil and steam sides to get rid of air locks while charging the system.

KORBA SIMULATOR

65

In a heat exchanger air locks reduce the effective heat transfer area and thus the heater efficiency. The vapour locks may also lead to eratic performance of the equipment and severe vibration. Air venting also helps to avoid the chances of forming fuel vapour air mixtures inside the system. All oil lines are run with a slope of about 0.3 - 0.5 % towards drain. Each section of oil line is provided with a drain valve or plug at the lowest point. All drain valves are normally kept closed during operation. When the oil system is taken out of service for a long duration, then it is necessary to open the respective drain the valves and drain the fuel oil when hot. Portable drip trays are provided near the drain points. HEAT TRACING OF HFO LINES The HFO being high viscous and having high pour point, the HFO lines are steam traced by running a small bore steam pipe along side and lagging together by insulation. The equipment like strainers and pumps have steam heating casing. This prevents loss of heat and eventual solidification of HFO in any section of the HFO piping. The formed condensate is let out through steam traps at the end of each tracer or heat jacket. Warming of the HFO lines and equipment like strainers and pumps before charging with oil is essential for easy flow and melt any solidified oil traces left behind during the prior shutdown. Also during shutting down each line or equipment heating helps in draining the system effectively. Sometimes the trace heating is continued even during normal burner operation to make up for radiation heat loss from the heated HFO so that the oil temperature does not drop. This may have to be practised only during the winter days. HFO PUMPING SYSTEM: Pressure Maintaining cum Regulating Valve The screw pump is a constant quantity pump and when only a small quantity of oil is fired, the excess oil from the constant quantity pump should be by-passed. This is done automatically by pneumatic operated, pressure maintaining cum regulating valve by by-passing the excess quantity through the return oil line to storage tank. The delivery pressure of oil is maintained constant at the pump outlet, whatever be the quantity of oil fired. Set the pressure control valve for maintaining adequate and constant pressure at the upstream of the HFO flow control valve at maximum firing rate.

KORBA SIMULATOR

66

KORBA SIMULATOR

67

KORBA SIMULATOR

68

The flow control valve upstream pressure required is the sum of the following at maximum firing rate:

• Oil pressure at the gun inlet.

• Static head between flow control valve and top level of burners, and frictional pressure drop in these lines.

• Flow control valve pressure drop; for best turndown.

HFO SUCTION STRAINERS Oil suction strainers are essential to prevent mechanical impurities reaching the small clearances and intricate passages in the screw pump. Basket strainers of 500 micron filter mesh are provided, with vent valve and drain valve. The running pump will starve of oil if the pressure drop across the suction strainer exceeds the allowable limit, and will get damaged. When the pressure drop across the operating strainer reaches about 0.3 kg/sq. cm (corresponding to 50 % clogged status), operation is switched over to the standby section of the Duplex filter. The clogged filter element should be cleaned without delay. The strainers are provide with alarming switches to indicate the operator when the cleaning of the filter is due. HEAVY FUEL OIL HEATING SYSTEM H. F. O. STEAM HEAT EXCHANGER Type U. Tube, Hair pin type, oil on shell side,

condensing type.

No. off 3 Nos. (including 1 no. standby)

Heater Area 40 Sq.M

Oil flow rate 680 lpm

Oil temperature range 150OC (oil)

Design pressure 6 Kg/cm2 (g)

Hydraulic Test Pressure 24 Kg/cm2 (g)

Design Temperature 250OC

KORBA SIMULATOR

69

GENERAL ARRANGEMENT There 150 % duty steam-oil heat exchangers and three duplex strainers are provided for operation in combination. The HFO temperature control valve and the trap station for heaters, steam jackets of strainers and line tracers are provided in the system. All these equipment are laid out on the floor. The drain points are to be suitably piped upto the drain pit from the drain trays. STEAM HEATERS AND STRAINERS The steam heaters are of fixed tube sheet, U tube type, with oil on shell side and steam on the tube side. The oil space is protected against exceeding of allowable pressure by low lifting spring loaded safety valve. The exchanger is equipped with the valves needed for air release and draining. The duplex basket type discharge strainers are at the heater outlet, with fine mesh of 250-micron filtration. The fine filtering prevents chocking of lines, valves and burner atomisers. The burner trip wearing rate is also reduced. When the pressure drop across the strainer exceeds about 0.5 kg/sq. cm (corresponding to 60 % clogged status), the standby strainer section is put into service and it is taken for cleaning. PULVERIZED COAL SYSTEM GENERAL The system for direct firing of pulverized coal utilizes pulverizers to pulverize the coal and a Tilting Tangential Firing System to admit the pulverized coal together with the air required for combustion (secondary air) to the furnace. As crushed coal is fed to each pulverizer by its feeder (at rate to suit the load demand) primary air is supplied from the primary air fans. The primary air dries the coal as it is being pulverized and transports the pulverized coal through the coal piping system to the cola nozzles in the windbox assemblies. A portion of the primary air is pre-heated in the bisector air heater. The hot and cold primary air are proportionally mixed, prior to admission to the pulverizer, to provide the required drying as indicated by the pulverizer outlet temperature. The total primary air flow is measured in the inlet duct and controlled to maintain the velocities required to transport the coal through the pulverizer and coal piping. the total

KORBA SIMULATOR

70

primary air flow may constitute from approximately 15 % to 25 % of the total unit combustion air requirement. The pulverized coal and air discharged from the coal nozzles is directed toward the centre of the furnace to form a firing circle. Fully preheated secondary air for combustion enters the furnace around the pulverized coal nozzles and through the auxiliary air compartments directly adjacent to the coal nozzle compartments. The pulverized coal and air streams entering the furnace are initially ignited by a suitable ignition source at the nozzle exit. Above a predictable minimum loading condition the ignition becomes self sustaining. Combustion is completed as the gases spiral up in the furnace. A large portion of the ash is carried out of the furnace with the fuel gas; the remainder is discharged through the furnace bottom into the ash pit. COAL FIRING SYSTEM COAL BURNERS

Type : Tilting Tangential

Make : BHEL

No. of Coal Burners feed by each

Pulveriser.

: 4

No. of elevation of Burners : 8

Total no. of Coal Burners : 32

Temp. of Coal Air Mixture : 66-77OC

Max allowable temp. of Burners : 850OC

Turn down ratio : 4 : 1

PULVERISERS

Type of pulveriser : 1003 x RP

No. of Mills/Boiler : 8

No. required for full load : 6

Base capacity of Mill : 68 Tonnes/hr for a pulverised fuel finess 70%

through 200 mesh with a raw coal of

grindability. Index 55 HGT and a total

moisture of not more than 8%.

KORBA SIMULATOR

71

COUPLING

Type : Gear type flexible coupling with spacer

assembly between mill & motor.

Supplier : M/S FAST's U.S.A.

Bearings : Antifriction bearings for main vertical shaft

assembly journal shaft assembly and worm

shaft assembly.

Cooling System : Immersed in oil bath

No. of Oil Coolers/Mill Two

Cooling water requirement for oil : Clean, colourless, and ammonia

contamination free cooling water of Qty given

below for the respective temp.

Oil Temp. oC : 21OC 32OC 41OC

: 106 140 170

Qty. Liters/Min. : 41 Litres/Min/Mill

Maximum Pressure : 10 Kg/cm2

Cooling water requirement for

journal Hydraulic system

: clean, colourless, and ammonia

Contamination free cooling water at 43oC.

Wear Surface : Grinding rolls, bullring segment and liners

made of High Quality Ni hard (Comb alloy-N)

casting, venturi vanes and classified Cone

inner surface are lined with ceramic liners.

Life of rolls/grinding in his rings : 4000/8000

Speed of pulveriser (RPM) : 42

Normal capacity with design coal : 52.5(T/hr)

Max capacity with design coal. : 61.00(T/hr)

Normal capacity with design coal : 56.6 T/hr)

Max capacity with worst coal : 60.00 T/hr)

Max crushed coal size the:

mill can accept.

: 30.00(MM)

RAW COAL FEEDER :

Type : SECO - 36" Gravimatric (with Mechanical

Weighing

KORBA SIMULATOR

72

Capacity : : Minmum - 4300 kg/hr.

: Maximum - 74800 kg/hr.

Supplier : : SECO. U.S.A.

Nuclear Monitor : 2 in No. (Upper and Lower)

COAL VALVES

TYPE LOCATION

36" Pressurised Coal Valves

limit torgue operated : At Raw Coal bunker outlet

CHAIN operated : At Raw Coal feeder inlet.

BUNKER SHUT OFF GATES:

Manufacturer : Stock Equipment Co.

Type : Rack and pinion (pressurised)

Motor rating : 0.5 KW

Material of gate : Stainless Steel/Besalt lined

COMBUSTION OF PULVERIZED COAL IN TANGENTIALLY FIRED FURNACES The velocity of the primary air and coal mixture within the fuel nozzle tip exceeds the speed of flame propagation. Upon the nozzle tip the stream of coal and air rapidly spreads out with a corresponding decrease in velocity, especially at the outer fringes where eddies form as mixture occurs with the secondary air. Here flame propagation and fuel speeds equalize, resulting in ignition. As the stream advances in the furnace, ignition spreads until the entire mass is burning completely. The speed at which the air and coal mixture ignites after leaving the windbox nozzles depends largely on the amount of volatile matter in the fuel. Heat released by oxidizing the volatile components in the coal accelerates of the fixed carbon to its ignition temperature. The key to complete combustion consists of bringing a successive stream of oxygen molecules into contact with carbon particles, the smallest of which are relatively large by comparison with the oxygen molecules. As combustion of the carbon progresses it becomes increasingly difficult to bring about contact with the diminishing oxygen supply in the limited time available, which for this type of firing is in effect greater due to the longer travel taken by the gases. The cyclonic mixing action that is characteristic of this type of firing is most effective in turbulently mixing the burning coal particles in a constantly changing air and gas atmosphere. As the main part of the gases spiral upward in the furnace, the relatively dense solid particles are subjected to a sustained turbulence, which is effective in removing the products of combustion from the particles, and in assisting the natural diffusion of oxygen through the gas film that surrounds the particles.

KORBA SIMULATOR

73

PULVERIZERS The pulverizer, exclusive of its feeder, consists essentially of a grinding chamber with a classifer mounted above it. the pulverizing takes place in a rotating bowl in which centrifugal force is utilized to move the coal, delivered by the feeder, outward against the grinding ring (bull ring). Rolls revolving on journals that are attached to the mill housing pulverize the coal sufficiently to enable the air stream through the pulverizer to pick it up. Heavy springs, acting through the journal saddles, provide the necessary pressure between the grinding surface and the coal. The rolls do not touch the grinding rings, even when the pulverizer is empty. Tramp iron and other foreign material discharged through a suitable spout. The air and coal mixture passes upward the classifier with its deflector blades where the direction of the flow is changed abruptly, causing the coarse particles to be returned to the bowl for further grinding. The fine particles, remaining in suspension, leave the classifier and pass on through the coal piping to the windbox nozzles. FEEDERS The raw crushed coal is delivered form the bunkers to the individual feeders, which, in turn feed the coal at a controlled rate to the pulverizers title “Gravimetric Feeders” given at the end of this chapter. In order to avoid overloading the pulverizer motor due to overfeeding, an interrupting circuit should be used to reduce the coal feed it the motor should become overloaded and to start the coal feed again when the motor load becomes normal. For details refer to Pulverizer instructions on its operation. PULVERIZED COAL DRYING For satisfactory performance, the temperature of the primary air and coal mixture leaving the classifier should be kept at approximately 77OC for our coals. Too low a temperature may not dry the coal sufficiently; too high temperature may lead to fires in the pulverizer. The outlet temperature must not exceed 90OC any case. The moisture content of coals varies considerably. Therefore the best operating conditions for an particular installation must be determined by experience. The location of dampers, shutoff gates and valves generally utilized. The hot air control damper and the cold air control damper regulate the temperature entering the pulverizer, by proportioning the air flow from the hot air and cold air supply ducts. These dampers also regulate the total primary airflow to the pulverizer. The hot air shutoff gate is used to shutoff the hot air to the pulverizer. The hot air gate drive must be interlocked with the pulverizer motor circuit so that the gate will closed any time the pulverizer is not in service. It must also be interlocked with the

KORBA SIMULATOR

74

temperature controller to effect closing of the hot air gate when the pulverizer outlet temperature exceeds 900C. The pulverizer discharge valves, the cold air shutoff gate and the seal air shutoff valves are always kept wide open. They are closed only when isolation of a pulverizer or feeder is required for maintenance. Pulverizer discharge valves are also closed on loaded, idle pulverizers when other pulverizers are being restarted after an emergency fuel trip. An adequate supply of clean seal air for the pulverizer trunnion shaft bearing, etc., normally is assured by installing two booster fans and a filter in the seal air system. One fan normally runs continuously, however it may be isolated for maintenance by closing its inlet shutoff damper. The filter in this system is an inertial separator type which discharges approximately 90 % of its input as clean air. A bleed off system, with a control valve, will control the amount of air being bled from the filter, so that the differential pressure between the filter air outlet and the filter bleed air outlet is zero. The control valve should be installed so the valve fails open with a loss of instrument air. The coal pipe seal air valve is utilized to admit seal air to the coal pipes for cooling when the pulverizer is isolated. The seal air valve is open whenever the pulverizer discharge valves are closed a vice versa. Primary air velocity requirements in the pulverizer and coal piping preclude wide variations in system airflows. Therefore a constant airflow is maintained over the entire pulverizer load range. The air flow should be low enough to avoid ignition instability and high enough at avoid setting and drifting in the pulverized coal piping or excessive supillage* of coal form the pulverizer through the trap iron spout. NOTE: Coal spillage may also be caused by overfeeding, insufficient heat inputs for drying, too low a hydraulic pressure on the rolls or excessive wear of the grinding elements. PULVERIZER COAL PIPING Each pulverizer supplies an entire elevation of windbox nozzles. By distributing the fuel in this fashion a balanced fire is maintained regardless of which pulverizers are out of service. Orifice plates are installed in the coal piping leaving the pulveriers, to compensate for unequal resistance to flow due to different lengths of piping to the windboxes.

KORBA SIMULATOR

75

GRAVIMETRIC FEEDERS MAIN FEEDER COMPONENT DESCRIPTION The STOCK Model 7736 gravimetric feeder is designed to supply 4366 to 76,408 Kgs. of coal to the pulverizer per hour while operating on a 415 volt, 3 - phase, 50 Hertz power supply. Operation of the principal feeder components is described in the text below. FEEDER BODY Feeder design exceeds NFPA Code 85F requirements and will withstand an explosion pressure of 35 KG/cm2. All parts in contact with active coal flow are fabricated of type

304 stainless steel. Side skirting is provided to contain the coal on the belt and a levelling bar near the feeder inlet shears the coal column to form a profile conductive to maximum weighing accuracy. Dust-tight doors are provided at both ends and each side of the feeder for access to critical components. Bullseye viewing ports in the doors permit observation of the feeder interior during operation. A work light mounted above each end door is designed to allow bulb changing from outside the feeder. BELT AND DRIVE SYSTEM The feeder belt is supported by a machined drive pulley near the outlet, a slotted take-up pulley at the inlet end, six-support roller beneath the feeder inlet, and a weighted idler in the middle of the feeder. A counter weighted scraper with replaceable rubber blade continuously cleans the carrying surface of the belt after the coal is delivered to the outlet. Proper belt tracking is accomplished by crowning the take-up pulley; in addition, all three pulley faces are grooved to accept the molded V-guide in the belt. The pulleys are easily removable for belt changing and bearing maintenance. Belt tension is applied through downward pressure exerted by the tensioning idler on the return strand of three belts. Proper tension is obtained when the round protrusion at the centre of the tension roll is in line with the centre indicator mark on the tension indicator plate. The tension roll indicator is found on the drive motor side of the feeder and is visible through the viewing port in the tension roll access door. Tension adjustments can be made with the feeder operating or at rest by turning the two belt take-up screws which protrude through the inlet end access door.

KORBA SIMULATOR

76

NOTE : CHANGES IN HUMIDITY OR TEMPERATURE MAY CAUSE VARIATIONS IN BELT LENGTH BELT TENSION SHOULD ALWAYS BE MAINTAINED WITHIN THE TWO EXTREME MARKS ON THE TENSION INDICATOR PLATE. WEIGHT SENSING AND CORRECTION SYSTEM COMPONENTS Coal weighing is performed on a span of belt downstream of the inlet, which is defined by 2 weigh span rollers. Located midway between these rollers is a weighing roller connected to a weigh lever. The weighing roller is free to move vertically, in order to sense the weight of coal on the weight of coal on the weigh span, but, is held in horizontal position by drag links on each side of the feeder. The weighing roller is equipped with levelling screws to adjust its height relative to the weigh span rollers. Coal is deposited on the belt beneath the inlet and formed into a profile conductive to accurate weighing by side skirts and by an adjustable levelling bar on the downstream end. In operation, the coal delivered to the feeder will vary in density, causing the weighing roller to move the weigh lever into an unbalanced position. If the coal is heavy, the overweight correction switch in the balance switch assembly will be activated, causing the weight correction motor to adjust the levelling bar downward. Coal height will be decreased to bring the weigh lever into balance. In an under-weight condition, the levelling bar is adjusted upward to increase the coal height. The weight correction gearmotor is of the constant speed reversing type, and positions the levelling bar to maintain a constant weight of coal per unit length of belt. Operation is controlled by two electronics cards, the data conversion card and the balance switch card is energized whenever the weight correction motor is energized: DS3 during an under weight correction and DS5 during an overweigh correction. To avoid unnecessary weight corrections for momentary transients, a pulse must be received from the data conversion card once per turn of the drive pulley at the same time that the balance switch indicates that there is a weight discrepancy before a weight correction signal is transmitted to the weight correction motor. A weight correction timer determines the length of time allowed for weight correction. This time interval is a function of drive pulley speed: the system enters a 12-seconds correction mode for each drive pulley speed is greater than 5 rpm, the system will operate the levelling bar continuously for as long as a correction signal is received. The balance switch assembly, located in the weighing compartment, is responsible for generating the weight correction signal when the weigh lever connected to it is no longer in balance. The balance switch assembly consist of 3 optical switches attached to a printed circuit board and mounted in an enclosure with a clear plexiglass cover. When the weigh lever is in balance, all, 3 optical switches are covered by a shutter. Uncovering the outboard switch in either direction generates the underweight or overweight correction signal. Uncovering either outboard switch plus the centre switch indicates a serious weighing discrepancy has arisen and generates the alarm signal.

KORBA SIMULATOR

77

A second constant speed reversing-type gearmotor is supplied to calibre the feeder weighing system. When the feeder mode selector switch (SSF) is in the CALIBRATE position, this motor can be energized to drive the poise weight on the weigh lever in the direction necessary to return the weigh lever into balance. With the weigh lever balanced properly, the correct weight of coal will be delivered by each revolution of the drive pulley. CLEANOUT CONVEYOR Two 18" wide strands of malleable iron drag chain with alternately spaced wing links are used to automatically clean coal from the bottom span of the feeder. This prevents interference with the belt and removes stagnant coal, which may otherwise ignite spontaneously. The sources of coal in the cleanout area may be: coal falling from the belt scraper, coal dust setting out of the air, coal removed from the self-cleaning take-up pulley, or coal blown off the belt by an improperly - adjusted seal air flow. The cleanout conveyor is driven by a 1/4 horsepower, totally enclosed, non-ventilated General Electric drive motor with tropicalized insulation through a reduction gearbox to an operating speed of slightly greater than 2 feet per minute. The cleanout conveyor is operated continuously with feeder operation to keep the coal in the bottom pan of the feeder at a minimum, since this coal is not weighed and will introduce an error into the coal feedrate. Continuous operation also prevents a corrosive build up on the links, which may, after long idle periods, cause binding of the links and subsequent drive overload. ELECTRICAL CONTROLS Electrical controls for the gravimetric feeder are housed in a remote located power cabinet assembly and in a control cabinet assembly mounted to the feeder adjacent to the drive motors. Three principal components in each assembly are described in the following text. DIGITAL TACHOMETER SYSTEM DESCRIPTION The digital tachometer is a special-purpose instrument for use in the calibration and maintenance of gravimetric and volumetric feeders. It convers an input frequency generated by the tachogenerator in the feeder motor or the output of a 40-or 60- tooth wheel and reluctance pickup input to the feeder tachometer into a motor rpm reading. The instrument is fully protable, self-contained, and designed to withstand sever use. It is 100 % solid state and has no internal adjustments. The instrument consists of an input amplifier, a phase-lock loop, a universal counter I.C., and an LED display assembly. All I.C.s are CMOs and the counter is designed to operate up to 50 KHz. The input to the amplifier is protected by clamping diodes, which do not allow the input to the op amp to exceed the limits established by the power supply. The out put of this op amp is a square wave, which is connected to a

KORBA SIMULATOR

78

Schmitt trigger gate which can be used wither as a direct input to the universal counter when the instrument is measuring frequency, or as an input to a phase-lock loop when the instrument is being used as a tachometer. The phase-lock loop is used to multiply the input frequency by either a factor of 50 or a factor of 15 to produce 600 pulses /revolution. The output of the phase-lock loop is then connected to the universal counter input when measuring rpm and, in this manner, ± .1 of an rpm resolution is obtained. The universal counter is a CMOs chip manufactured by Intersil, using a 10 MHZ crystal as a time base reference and containing all the circuitry necessary to do the counting, decoding, and driving of a 5-digit, 7-segment, multiplex display. The selector switch, utilizing an input from the digit select line, does location of the decimal point. Four AA nickel cadmium batteries provide power for the instrument supply. Charging power is provided from a standard AC/DC adapter charger with an output or 8 to 10 V @ 100 mA current. A diode is provided to protect the instrument in case a charger or reverse polarity is inserted into the battery-charging jack. CIRCUIT DESCRIPTION The instrument consists of a power supply, input amplifier, a pulse-to-rpm converter, frequency counter, a display, and a function selector switch. The function of the input amplifier is to accept data from the various input source and normalize the signal amplitude to make it compatible with the instrument logic. The pulse-to-rpm converter processes the data from the input amplifier by the use of a phase-lock loop and converters frequency counter measures the incoming frequency, utilizing a 10 MHz Crystal for a time base. The frequency counter also drives the displays, whose digits are 4.4 mms in height and are operated in the multiplex mode. The power supply consists of 4-nickel cadmium AA batteries and a diode-resistor circuit to prevent a change current. The function selector is a 4-position switch to select the operating mode of the instrument. PULSE-TO -RPM-CONVERTER The circuit consists of a phase-look loop, U4, and a divider, U5. The phase-lock loop (PLL) is a circuit element designed to lock the output of an internal frequency generator to the input frequency. If this frequency is divided down by a counter, the output of the voltage control oscillator (VCO) would be the input frequency times the divider of the counter. The phase -lock loop measures the difference between the incoming frequency and the output of the VCO divided by the counter and generates a signal proportional to the difference. This signal, or error, operates a voltage control oscillator. Resistors R11, R12 and C4 are a filter for the output of the phase comparator. The operational range of the voltage control oscillator is established by capacitor C%. The output of the VCO is connected to counter U5, whose outputs are used to divide the output frequency of the VCO by either 50 (when the switch is in the RPM/12 position) or 15 (when the switch is in the RPM/40 position). This, in effect, multiplies the input frequency into the PLL by either 50 or 15, depending on the selector switch position. Thus, in the RPM/12 position, a 320 Hz input will be

KORBA SIMULATOR

79

displayed as 1800 rpm. With the selector switch in the RPM/40 position, an input of 1200 Hz will be displayed as 1800 rpm. Because the phase-lock loop increases the resolution of the instrument, a decimal point is added to the display and the shaft is indicated with a resolution of .1 rpm. If the input is 60 pulses per shaft resolution, the phase-lock loop is not used and the resolution of the instrument is in .1 rpm if measuring speed or 1 Hz measuring frequency. BUNKER OUTLET VALVE VALVE GATE The gate is fabricated in a winged “U” design to keep its supporting rollers, racks and pinions completely out of the coal stream. This design minimizes potential corrosion from moisture in the coal. To assure closure of the gate without cocking or binding, two pinions on the operating shaft engage ladder racks, which extend down each side of the gate. The pinions are located above the racks to provide positive tooth engagement. To keep maintenance at a minimum, the ladder racks and pinions are designed with self-cleaning capability; and the gate is supported in individually greasable roller assemblies to further ensure proper tracking. VALVE OPERATOR The valve is operated by a Limitorque Model SMB-00 valve operator, which includes a .497 KW, totally enclosed, non-ventilated motor wired for 425 volt, 3-phase, 50-Hertz operation. The motor is rated for a 15-minute duty cycle. A space heater is provided to dissipate moisture from the operator in damp locations. Between the Limitorque operator and the valve body, a 3:1 gear reducer is provided. The valve operator is equipped with a torque switch wired into the motor control circuit to stop the operator in a full open (ACWT) or full closed (CWT) position when a predetermined amount or torque output is developed. A limit switch is provided to energize the red OPEN and green CLOSED position indicators when the valve has reached its limits of travel. Valve controls include momentary OPEN and CLOSE pushbuttons, as well as a STOP pushbutton to de-energize the operator in intermediate positions. The indicators and pushbuttons are mounted to a NEMA 12 control station. In the event of a power failure, a pocket sheave-type handwheel with hand chain is provided for emergency manual override of the valve operator. The operator has an automatic handwheel declutching arrangement in which a declutch lever must be pulled down to mechanically disconnect the electric motor before manual operation takes place. The valve operator will then remain in manual operation indefinitely until the electric motor is energized, which causes tripper cams mounted on the worm shaft to release the clutch ring and keys from their manual positions and engages the motor. When

KORBA SIMULATOR

80

the handwheel is turned manually, the valve motor does not rotate; and when the motor is in operation, the handwheel does not turn. NOTE : DO NOT DPRESS THE DECLUTCH LEVER DURING VALVE MOTOR OPERATION TO STOP VALVE TRAVEL. USE THE STOP PUSHBUTTON FOR THIS PURPOSE. SECOAL NUCLEAR MONITOR: The SECOAL Double Nuclear Coal Monitor is designed specifically for application in central power generating stations and industrial boiler houses for early detection of coal voids in industrial boiler houses for early detection of coal voids in the downspout preceding the coal feeder. The basic physical principle used in the SECOAL system is that of detecting gamma radiation generated by a nuclear source when directed radially through the downspout. The radioactive source is located on one side of the downspout and the detector is located on the opposite side. When the downspout is full of coal, the detector will sense a certain level of radiation. When a partial void or pocket exists in the coal a greater level of gamma radiation will reach and the sensed by the detector. Electrical pulses are emitted by the detector unit in proportion to the radiation level sensed. These pulses are statistically analysed by the monitor’s electronic circuitry which can distinguish between these radiation level and thus determine that a void or pocket has occurred in the downspout. The double SECOAL nuclear utilizes two sources and detector assemblies: one located at the STOCK pressurised coal valve and the second located in the downspout above the feeder. When the upper unclear monitor detects a void, indicating a pluggage or an empty bunker, it activates a bunker vibrator in an attempt to restore coal flow. If the void remains, and is then detected by the lower nuclear monitor, the feeder is de-energized and an alarm condition is annunciated. The location of the lower nuclear monitor is critical in that the feeder must be deenergized in time to preserve the head seal above it during a loss of coal condition. Since the pulverizer operates at greater than atmospheric pressure, the feeder supply it becomes pressurised accordingly. And because the bunker outlet is essentially at atmospheric pressure, the head seal is necessary facilitate coal flow into the feeder. The head seal is the actual column of coal in the downspout, which, over its height, evenly dissipates the pressure in the feeder to atmospheric at the bunker outlet. It must be of sufficient height to prevent fluidization of the coal, caused by an excessive pressure drop over too small a portion of column. If coal flow in the downspout is re-established before the column is drawn down to the level of the lower nuclear monitor, the bunker vibrator is de-energized and the feeder will continue to operate normally pending a future loss of coal detection. When

KORBA SIMULATOR

81

properly calibrated, the SECOAL system can reliably sense a void and produce an alarm for the equivalent of a 6 inch loss of coal as viewed radially through the downspuot. SOURCE AND DETECTOR ASSEMBLIES The radioactive source material, Radium 226, is hermetically sealed in a multiple capsule and placed in a lead-shielded, heavy steel source containers mounted on one side of the valve or downspout at the eleveations at which void detection is required. The detector assemblies, mounted opposite the source containers at the same elevation, are Geiger - Muller tubes embedded in plastic inside an aluminium shell. A calibration handle at the base of the main frame is used to pivot each source and detector as a unit toward a series of three stationary calibration blocks. When rotated to the CALIBRATE position, the radioactive beams are directed through a set of steel plates having a known absorption rate, corresponding to the absorption rate of the downspout, considering its size and material of construction. Calibration consists of adjusting a potentiometer to effect the proper monitor sensitivity. Two additional sets of calibration blacks are provided for each detector to simulate an EMPTY and FULL downspout. Rotation of the source and detector in turn to these positions, through proper energization and sequencing of the READY and ALARM indicating lights, verifies the calibration procedure. Calibration can thus be performed with or without the feeder in operation, and without emptying coal form the downspout. NOTE : THE SECOAL NUCLEAR MONITOR CAN DETECT AND BE ACTIVATED BY STRAY RADIATION GREATER THAN 0.15 mR/hr. WHEN ANY KIND OF X-RAY WORK IS GOING ON AT THE PLANT WITH RESPECT TO PIPING OR CONSTRUCTION, PLACE THE CALIBRATION HANDLE IN THE CALIBRATE, POSITION, POSITION TO AVOID FALSE ALARM INDICATIONS. The belt drive system consist of a Louis -Allis, 5 HP variable speed DC shunt wound motor with a speed range of 100-1750 rpm, The motor is housed in a totally-enclosed, non-ventilated enclosure with class II epoxy coated insulation with-tropical protection, server duty house down provisions, and a 150 watt space heater wired for 240 V AC operation. The motor operates through a multiple reduction gearbox to a total reduction of 149.6 : 1. A reluctance type magnetic sensor is provided on the motor drive to detect motor speed. This data is used for motor speed control feedback information, for zero speed detection (i.e. motor speed less than 60 rpm), for derivation of a pulse signal for data logging, and for feeder weighing control information. One revolution of the

KORBA SIMULATOR

82

feeder belt drive pulley delivers a predetermined weight of coal, regardless of its density, to the outlet. A signal from the combustion control system, operating through the speed control, regulates the belt drive motor speed, and thereby regulates the coal federate. A paddle type alarm is mounted above the centre of the belt to detect the presence or absence of coal on belt. The alarm system consists of a stainless steel paddle mounted on one end of a horizontal shaft and a dust tight switch housing on the other end. Multiple single pole switches, depending on the number of functions requiring control, are mounted in the switch housing. The switches are actuated by adjustable cams mounted on the end of the shaft inside the switch housing; loss of coal on the belt results in a contact closure of limit switch LSFB. This switch can be used to stop the belt drive motor, start a bunker vibrator, or simply to indicate a loss of coal to the control room, as directed by the customer. This contact closure also prevents weight correction and operation of the total coal integrator when there is no coal on the belt and prevents calibration when there is coal on the belt. SCANNER AIR SYSTEM The scanner viewing heads are located in the burners and they are exposed to furnace radiation continuously. The scanner heads cannot with stand high temperatures that will arise due to this exposure. A constant cooling air is required around the scanner heat to cool it to a safe working temperature to ensure a reliable operation and long life. The scanner head cannot be exposed to a continuous temperature of 175° without cooling air. A continuous cooling air quantity of 80 mm3/hr per scanner is required for effective cooling. The cooling air temperature shall be below 650C. When the boiler is shut down it is necessary to keep the cooling air on until the furnace cools down to a safe temperature The scanner air is supplied to the scanners from the header through the flexible hoses. It is important to ensure that no damage to the hoses are left unrepaired and 'all leakage are attended without fail. The scanner cooling air is supplied through a fine filter to clean out any suspended dust particles. The filter is provided with a switch to indicate the filter plugged condition. The filter "can be changed within a short time. The filter is made of a catridge construction, which can be pulled out easily. The spare filter element should be made available readily near the filter. During the filter changing period it is permitted to use unfiltered air. The filter assembly is also provided with a 'no filter element' alarm switch which will indicate the operator that the filter element is not in line.

KORBA SIMULATOR

83

The air pressure after the filter is monitored by a differential pressure switch which will give an alarm when the pressure difference goes below the safe level. Two scanner fans are provided to supply the cooling air. One fan will be operating with AC while the stand by fan can be operated by station DC power supply. The stand by DC powered fan starts automatically when there will be an AC power supply failure. Pneumatically operated dampers are provided at the outlet of the fans. One f~n is operated with AC and other by DC supply. The damper of the operating fan will be in closed condition. Two number hand operated dampers are also provided at the fan suctions. These are nornlally in open condition only and they are used to isolate the fans during the inspection or maintenance of the fan.

The suction of the fans are taken from the cold air duct after the FD fans. When the FD fans trip the suction is taken from the atmosphere through a DC operated pneumatic operated shut off damper. SCANNER AIR FAN Nos. 2 No. one is A.C. driven Other.is

D.C. driven 100% standby

Type of fans Radial

Boosting pressure 254 mmwc

Design capacity of each fan 3600 M3/hr

Design temp. 50°C

Motor rating 5 KW

Speed 1500 rpm

KORBA SIMULATOR

84

KORBA SIMULATOR

85

AIR AND GAS PATH GENERAL Two forced draught fans and two primary air fans handle airflow to the boiler. The flue gas produced in the furnace is evacuated by three number of I.D. fans of which one ID fan is stand-by. This chapter contains descriptions of draught and air systems associated with the main steam generators, and of the major ancillary equipment’s used in these systems. SEAL AIR FAN Manufacturer : BHEL Type : NDM-6 No. of Fans per Boiler : 4 Mounting : Ground mounted Arrangement : Horizontal Shaft CAPACITY

Flow rate at 100% MCR (NM3/hr) : 10200

Flow rate at design point (NM3/hr) : 12800

Max flow rate the fan can handle (NM3/hr) : 13920

STATIC PRESSURE Pressure at 100% MCR (MMWC) : 390 Pressure at design point (MMWC) : 508 Source of air suply : PA Fan Discharge Normal Speed (RPM) : 2880 Power consumption at 100% MCR KW/Fan : 20 Impeller Dia (MM) : 670

PRIMARY AIR/MILL SEAL AIR SYSTEMS The primary air system supplies heated air to the coal mills to dry and convey pulverised coal to the furnace. Ambient air is drawn into the primary air ducting by two 50 % duty, motor driven axial reaction fans, each capable of providing sufficient air to support 60 % Boiler MCR.

KORBA SIMULATOR

86

The inlet to each fan is silenced and includes pneumatically operated guide vanes to control fan output. The position of guide vanes is controlled by the ‘P.A. header pressure control loop’ to maintain air pressure in the P.A. bus duct at a pre-set level. Air discharging from each fan passes first through a steam coil air pre-heater then through a motor operated guillotine gate into the P.A. bus duct. The motor operated isolating gates are manually operated from separated OPEN/CLOSE push button stations in the unit control room (UCB). The P.A. bus duct has four outlets out of which two direct cold air through the primary air heaters into the hot air cross over duct; two take cold air to the seal air fans and the hot air duct prior to the mill air flow venture. The primary air heater air inlet and outlet ducts are fitted with motor operated, biplane dampers, which are, operated form push buttons in UCB. Both inlet and outlet dampers are operated from UCB through separate push buttons. The hot air cross over duct extends around to each side of the boiler to form the hot air to mills ducts, both of which are branched to supply hot air to four coal mills. Each branch is fitted with identical equipment. Hot air first passes through a pneumatically operated isolating gate, then on through a motor operated regulating damper, and a flow venture into the coal mill. The hot air isolating gates are operated through the FSSS interlocks. During tripping of a mill hot air gate and damper will close automatically and cold air damper will open fully to provide cooling air to mill. The hot air regulating dampers modulate under the automatic control loops to maintain the required airflow to the mill under varying load conditions. Flow transmitters located about the venture provide a measured mill P.A. flow signal to automatic control loops. Cold air taken direct form the P.A. bus duct, is routed to each side of the boiler to form three cold air bus. A branch of cold air bus connects to three hot air ducting upstream of the flow venture and includes a motor operated regulating damper which modulates in response to the mill outlet temperature control loop to maintain the mill outlet temperature at a pre-set level. Two branches from the cold air bus deliver air to the mill for sealing purposes. Each branch has 2x100 % duty parallel mounted strainers (duty/standby) further connected to two mill seal air fans which boost the air pressure to maintain sufficient differential between P.A. and seal air. Each strainer is fitted with hand operated inlet and outlet dampers. Seal air fans have hand operated inlet dampers and pneumatically operated outlet damper. One is standby in each branch.

KORBA SIMULATOR

87

KORBA SIMULATOR

88

SECONDARY AIR SYSTEM The secondary air draught plant supplies the balance of air required for pulverised coal combustion air for fuel oil combustion and over flow air to minimise NOX production. Ambient air is drawn into the secondary air system by two 50 % duty, motor driven axial reaction forced draft fans with variable pitch control, each capable of providing sufficient air to support 60 % BMCR. Silencer is provided at the suction. Air discharging from each fan passes first through a steam coal air preheater then through a motor operated isolating damper into the secondary air bust duct. The isolating dampers are operated from separate push button stations in UCB. Flow is measured across the venture provided in the discharge ducts, by two transmitters, which feed their signal to the automatic total air control loop. This signal is added to the coal mill P.A. Flow signals then compared with the airflow demanded by the boiler load control loop. Any difference will cause the pitch angle to modulate towards the demanded flow. The F.D. bus ducts direct air through the two secondary air heaters into the cross over duct. The secondary air heater inlet and outlet ducts are fitted with motor operated biplane dampers, which are controlled from separate push button stations in the UCB. One other outlet from F.D. bus duct directs air into the scanner air fans. The cross over duct extends around to each side of the boiler furnace to form two secondary air to burner ducts. At the sides of the furnace, the ducts split to supply air to two corners, then split again to supply air to each of the nineteen burner/air nozzle elevations in the burner box. Each elevation is fitted with a pneumatically operated regulating damper, which is controlled by the Secondary Air Damper Control system to maintain optimum secondary air distribution for combustion with varying fuels and firing conditions. Five basic types of burner box dampers are used:

1. Coal / air dampers which admit air immediately around the pulverised fuel nozzle and hence are constituent in the primary stages of combustion.

2. Secondary air dampers, which admit air around the coal/air and P.F. nozzles

and hence are involved in the latter stages of combustion. These dampers will be controlled to maintain the desired differential pressure between the secondary air to burner and the furnace.

3. Oil/secondary air dampers, which generally fulfill the same requirements as

but with additional requirement of providing air for oil burning. When oil

KORBA SIMULATOR

89

burning is in progress, the associated damper will modulate according to oil header pressure.

4. Bottom tier secondary air dampers, which form part of the secondary air

system, but utilised to maintain clear conditions in the lower furnace.

5. Over fire air damper, which direct air over the coal flame to minimise Nox production.

FLUE GAS SYSTEM The flue gas draught plant draws hot flue gases from the furnace and discharges them to atmosphere through the chimney. During its passage to the chimney, flue gas is passed through an economiser and four air heaters to improve thermal efficiency, and through four electro-static precipitators to keep dust emission from the chimney within prescribed limits. The flue gas ducting starts from boiler down stream of the economiser and directs flow towards three primary and secondary air heaters. The primary and secondary air heaters gas inlet duct is fitted with biplane isolating dampers. The gas outlet ducts of all four air heaters are fitted with lower type regulating dampers. The outlet ducts of corresponding primary and secondary air heaters combine then discharge through a regulating damper, into the electrostatic precipitator common inlet duct which directs flue gas through four electrostatic precipitators into the ID fan common inlet duct. The inlet outlet ducts of each precipitator have motor operated guillotine gates. From the ID fan common duct, flue gas flows through two of three 50 % duty I.D. fans (one standby), each capable of supporting 60 % BMCR, into a common duct to the chimney. Each fan has a motor operated guillotine gate for isolation at the inlet. The outlet of two extreme fans has a similar gate whereas the outlet of middle fan, which bifurcates into two branches, is fitted with two guillotine gates. The fans are equipped with pneumatically operated inlet guide vanes and a variable speed control that are controlled by boiler furnace draught control loop to maintain furnace draught at a pre-set level. PRIMARY AIR FAN P.A. FAN MOTOR Manufacturer : M/S SIEMENS, West

Germany/BHEL

Motor type : SQ motor

Direction of rotation as viewed from non

driving end

: CCW

Standard Continuous rating at 10oC ambient

temp.

: 2300 KW

KORBA SIMULATOR

90

Rated voltage : 6600 V

Rating for specified normal Condition i.e.,

50OC ambient temperature

: 2100 KW

PERMISSIBLE VARIATION OF

Voltage : 6600V ± 10%

Frequency : 50 HZ ± 5%

Minimum permissible starting voltage : 80%

Rated speed at rated voltage : 1494 rpm

At rate voltage and frequency

Full load current : 212 A

No load current : 58.5 A

Starting time with minimum permissible

voltage of 80% of rated value:

Without fan : 1.5 sec

With fan : 13 sec

For Bearings

Type : NU-232 NU 228

6232 C4

Weight of Motor stator : 7500 kg

Motor rotor : 2100 kg

Recommended lubricant : Grease-Servogem 3 of IOC

Lub oil circulation system of P.A. fan

Tank capacity : 400 litres

Type : A112 MA-4F

Output rating (motor) : 4.00 KW

Supply (motor) : 415V + 10%, 3 Ph, Delta-

connection

Full load speed : 1500 rpm

Frequency : 50 Hz + 5%

Pump

Make : BORNEMAINN

Type and Capacity : E4V 045 KIF 215/1.363 dm3/s

Speed : 1450 rpm

KORBA SIMULATOR

91

Discharge pressure : 16 bar

Lube oil properties

Quality : L-TD 68

Pour point : 6OC or lower

Flash point : min 205OC

Water content : Less than 0.1g/100 lit. of oil

Contents/solid foreign matter : Less than 0.05 gm/100 lit. of oil

Viscosity at + 40oC : 61.2-74.8 mm2/s

Viscosity at + 20oC : max. 200 mm2/s

Viscosity at + 15oC : max. 0.9 gm/cc

P. A. FANS There are two primary air fans per boiler. The fan consists of the following components:

1. Suction bend, with an inlet and an outlet side pipe for volume measurements.

2. Fan housing with guide vanes (stage 1)

3. Main bearings (anti-friction bearings)

4. Rotor, consisting of shaft, two impellers with adjustable blades and pitch control mechanism.

5. Guide vane housing with guide vanes (stage 2)

6. Diffuser with an outlet -side pipe for pressure measurements.

Suction bend, fan housing and diffuser are welded structural steel fabrications, reinforced by flanges and gusets, resting on the foundation on supporting feet. The supporting feet are fixed on the foundation in such a way that they slide and without clearance at the sliding supports of suction bend and diffuser. On its impeller side, the suction been is designed as an inlet nozzle. Guide vanes of axial flow type are installed in the fan and guide vane housings, in order to guide the flow. Further more, the guide vanes are connecting the core and jacketing of the housing.

KORBA SIMULATOR

92

Suction bend and diffuser are flexibly connected to the fan housing via expansion joints. Fan and guide vane housing are horizontally split, so that the rotor can be removed without having to dismount the servomotor. Those parts of the pitch control unit, which are arranged in the guide vane and diffuser cores, are accessible through assembly openings. The fan is driven from the inlet side. The shaft runs in antifriction bearings. The main bearings are accommodated in the core of the fan housing. The impellers are fitted to the shaft in overhung position. The centrifugal and the setting forces of the impeller blades are absorbed by the blade bearings. For this purpose the blade shaft is held in a combination of radial and axial antifriction bearings. Each blade bearing is sealed off by means of several seals, in both directions (towards the inside and the outside). PITCH CONTROL UNIT 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 control parts. At pitch control, the translational movement of the servomotor piston is converted into a rotational movement of the blade shafts via adjusting levers, so that the blade angles are variable. The restoring movement of the blades results from their rotating mass. Weights are fitted to the blade shafts, which partly compensate for the restoring moments. These weights produce moment acting against restoring moments. These weights produce moment acting against the restoring moment of the blading, so that the adjusting components are relieved. The oil pressure in the servomotor maintains equilibrium with the residual moment. The oil-hydraulic servomotor can be connected to any control system. It can also be operated by hand. To initiate pitch control, the non-rotating control slide is moved in axial direction, This requires but little force (a few N). The control slide is hydraulically, centred. It moves on a liquid film practically preventing friction and wear.

KORBA SIMULATOR

93

The pitch control unit operated in accordance with the follow-up control principle. The control system outside of the machine initiates the actuating motion of the non-rotating control slide via rods (viz. the adjusting drive). The adjusting piston and, via the adjusting disc the blades, follow up each motion of the control slide. The control oil conveyed by the unit reaches the control slide at constant pressure. If the governor moves the control slide to the left and if the slide is kept in that position, the right control edge opens the admission to chamber 2. The pressure oil flows into this chamber and moves the adjusting piston to the left, until the control edges are in line again. Simultaneously, the oil out of chamber l flows via the control slide into the return piping to the oil reservoir. Analogously the same happens, if the governor moves the control slide to the right. The left control edge of the control slide will then open the admission to chamber 1 and the oil pressure will move the adjusting piston to the right, until a state of equilibrium is regained. The adjusting disc is firmly connected with the adjusting piston. It transfers the to and from movements of the adjusting piston via slide pads and levers as rotation movement to the blade shafts. Actuation of the second impeller blades is carried out in the same way synchronously via the adjusting bar. This hydraulic adjusting unit and the oil unit form one system. If the oil supply is interrupted, the blades will stay in their positions and there will be no interference with the pitch control. The pitch control impulse must through be interlocked via the control oil pressure. LOCK OUT Pitch control is feasible only if there is control oil pressure. This refers to hand and to automatic actuation. Blade positions are shown on graduated plate on machine. OIL SYSTEM The main bearings and the hydraulic servomotor are supplied with oil from a common oil reservoir. This has the advantage that for both the units the same oil can be used. It is recommended to useturbine oil with a viscosity of 61.2-74.8 mm sq./sat 40 deg. C.

KORBA SIMULATOR

94

Two oil pumps are mounted on the reservoir. One is operated as the main pump, whereas the other one is used as standby machine. The latter is started via the pressure switch, in the event the control oil pressure declines. Non-return valves prevent the pressure oil from flowing back to the reservoir through the pump being out of operation. The pressure in the system is set and maintained by the pressure limit valve. This valve causes the oil, which is supplied by the pumps, but is not required in the circuit (e.g. no adjustment is effected), to flow back to the reservoir without pressure. The return line must be let beyond the oil level in the reservoir. The oil supply system is also equipped with a pressure reservoir. It is mounted in the control oil piping in front of the oil cooler. The function of the pressure reservoir is to absorb pressure peaks occurring when starting and adjusting, as a result ot the response time of the pressure limit valve. It, thus serves as vibration damper. The oil is cooled in the oil cooler. The oil cooler is designed as double oil cooler. The thermometers-upstream and downstream of the cooler- indicate the cooling effect. In addition, a double resistance thermometer and a double contact thermometer are arranged downstream of the cooler. The oil filter is designed as a twin filter, which allows cleaning the filter insert during operation. The position of the reversing lever tells which filter chamber is in use. The filter has a differential pressure indicator which optically shows the degree of contamination and which releases an accoustic signal when a very high degree of contamination is reached. We would point to the fact that dirty filters are quite often the cause of the pressure decline of the lube and control oil. In most cases reversing the filter can stop the pressure decline. The disconnected filter chamber can be cleaned during operation. Behind the oil filter, the oil flow divides in a control and lube oil circuit. A throttle controls the oil pressure and the oil quantity in the lube oil circuit. Behind the throttle, the lube oil flows to the fan bearings. Local surveillance (at the oil supply unit) of the lube oil pressure is ensured by pressure gauges. The lube oil circuit and the main bearings are vented through the lube oil return line. therefore this line is not led below the oil level in the reservoir. The lube oil flowing back through the return line can be observed by means of a sight glass.

KORBA SIMULATOR

95

The control oil pressure is shown and monitored by contact pressure gauges. The control oil circuit is vented through the leakage oil return line. In this line as well as in the control oil return line sight glasses are installed allowing the observation of the returning oil. By means of the orifice a counter-pressure against the return flow is produced: pressure fluctuations in the system are thus largely reduced. FORCED DRAUGHT FAN F.D. FAN MOTOR Manufacturer : M/S SIEMENS, West

Germany/BHEL

Motor type : SQ motor

Direction of rotation as viewed from non

driving end

: CCW

Standard conditions rating at 40OC ambient

temp

: 1670 KW

Rating for specified normal condition i.e.

50OC

: 1400 KW

Rated voltage : 6600 V

Permissible variation of : 6600 V±10%

Voltage (volts) : 50 Hz ± 5%

Frequency (Hz)

Comined voltage and frequency : ± 10%

Minimum permissible starting Voltage : 80%

Rated speed at rated voltage and frequency : 994 rpm

At rated voltage and frequency

Full load current : 147 A

No load current : 42.5 A

Starting time with Minimum permissible

voltage of 80% of rated value Without driven

equipment coupled

: 2 second

KORBA SIMULATOR

96

F.D. FAN

Type : Axial reaction API-16/16

No. per Boiler : Two

Fan design rating

Capacity : 260.5 M3/sec

Medium Handled : Fresh air

Location : Ground level

Total head developed : 391 mmwc

Temp. of medium : 50OC

Specific wt. of medium : 1.046 kg/m3

Fan speed : 980 rpm

Type of coupling : Rigiflex form 11 size 2001

Fan wt. : 27.5 tonnes

Lubrication equipment

Fan lubrication : Forced oil circulation

Motor lubrication : Grease lubricated

Type of fan regulation : Blade pitch control

Bearings

Fixed bearings : Cyl. roller Brg NU 248

C3,Ang.contact Ball

Bearing 07248B

Expension bearings : Cyl-Roller Brg. NU 248C3

Cyl-Roller Brg. NU 248C3

Fan reserve

Fan flow : 36.7%

Fan pressure : 45.3%

Recommended lubricant : OIL/52 lit/min per

motor/IOC Gr.Servo-prime

46 or equivalent

Cooling water requirement for CACW motor

Quantity required M3/hr : 70.6 per motor

Max. permissible inlet water temp : 38OC

KORBA SIMULATOR

97

Pressure of water at inlet to coolers

: Upto 7.5 bars over

pressure

Outlet temp. of water at full load (anticipated value): 43OC

Wt. of motor stator : 5300 kgs.

Wt. of motor rotor : 5100 kgs.

FORCED DRAUGHT FANS There are two forced draft fans per boiler. Each fan consists of the following components :

1. Suction bend, with an inlet-side pipe for volume measurements.

2. Inlet housing

3. Fan housing, with a pipe for volume measurements

4. Main bearings (antifriction bearings)

5. Impeller with adjustable blades and pitch control mechanism.

6. Guide vane housing with guide vanes

7. Diffuser, with an inlet-side pipe for volume measurements. Suction bend, inlet housing and diffuser are of one-part, fan and guide vane housings of two-part design. Suction bend, fan housing and diffuser are structural steel fabrications, reinforced by flanges and gussets, resting on the foundation on supporting feed. Fan and inlet guide vane housings are split horizontally in such a way that the rotor can be removed while the servomotor remains in place. The impeller-side end of the suction bend is designed as inlet nozzle. The fan is driven from the inlet side. The shaft runs in the antifriction bearings. The main bearings are accommodated in the core of the fan housing. The impeller is fitted to the shaft in overhung position. The Fan shaft is designed in such a way that the maximum operating speed is below the critical speed. The centrifugal and the setting forces of the impeller blades are absorbed by the blade bearings. For this purpose, the blade shaft is held in a combination of radial and axial

KORBA SIMULATOR

98

antifriction bearings. Each blade bearing is sealed off by means of several seals, in both directions (towards the inside and the outside). Those parts of the pitch control unit, which are arranged in the guide vane and diffuser cores, are accessible through assembly openings. The sliding supports of the feet of suction bend and diffuser are fixed on the foundation in such a way that they slide without clearance. Bearings The rotor is accommodated in cylindrical roller bearings. In addition, an angular contact ball bearing is arranged at the driving side in order to absorb the axial thrust. Double contact tele-thermometers and double resistance thermometers are fitted to monitor the bearing temperature. These thermometers must be connected to signalling instruments on the site. Pitch Control Unit Details are same as given in Section 2.14 on Primary Air Fan. Oil System Details are same as given in Section 2.14 on Primary Air Fan. INDUCED DRAUGHT FAN I.D. FAN MOTOR Manufacturer : SIEMENS, West Germany,

BHEL

Type : SQ-motor

Rated voltage : 6600V

Direction of rotation as viewed from non driving

end

: CCW

Rating : 3400 KW

Water inlet temp. : 38OC

Permissible variation of

Voltage (volts) : 6600V ±10%

KORBA SIMULATOR

99

Frequency (Hz) : 50 Hz + 5%

Combined voltage and frequency : + 10%

Minimum permissible starting

Voltage : 80%

Rated speed : 596 rpm

At rated voltage and frequency

Full load current : 379 A

No. load current : 139 A

Starting time with minimum permissible voltage of

80% of rated

Without fan : 1.7 second

With fan : 2.5 second

Bearings

Type:

Drive end : Sleeve air cooled

Non drive end : Sleeve ring

I.D. FAN

Type : Radial fan double suction

NDZV 45 sider

Make : BHEL

Orientation : Suction at 45o delivery at

bottom horizontal

Medium handled : Flue gas

Location : : Ground level

No. off per Boiler : 2+1 standby

Fan design rating

Capacity : 533.8 M3/sec

Total head developed : 409 mmwc

Temp. of medium : 150OC

Sp. wt. of medium : 0.796 kg/m3

Speed : 550 rpm

KORBA SIMULATOR

100

Reserve

Fan flow : 25.6%

Fan pressure : 44%

Fan drive coupling

Type of fan coupling : Hydraulic

Fan coupling (make) : M/s Voith, West Germany

Fan weight : 100 tonnes

Lubrication

Fan lubrication : Stand oil lubrication

Motor lubrication : Stand oil lubrication

Type of fan regulation : Variable speed through

Hydraulic coupling plus

damper control.

Bearings

Fixed bearing : Dodge sleeve oil RT-20

size-9"

Expansion bearing : Dodge sleeve oil RT-20 size

-9"

INDUCED DRAUGHT FANS There are three induced draught fans per boiler, two operating and one standby. The induced draught fans are NDVZ type. NDZV fans are single -stage, double-inlet centrifugal fans. Principal fan elements: housing, inlet dampers, rotor with bearings and shaft seal. The box-section scroll with the inlets and the outlet is of two-part welded design. The supporting structure of the housing is formed by parallel lateral walls that are welded to the coating surfaces of the scroll and of the inlets. Special supporting bolts, ribs and reinforcements stiffen the welded structure. The inserts welded into the box-section scroll and into the inlets guide the flow and moreover reinforce the components. Scroll housing, inlets and outlet consist of rectangular sections and are equipped with man-holes. The bottom part of the housing rests on claws on the foundation. The scroll skin is equipped with a wear protecting coating on the inside. The inlet dampers are accommodated in the inlet damper housing they are commonly adjustable externally.

KORBA SIMULATOR

101

The rotor consists of a centre disc and two cover discs that are reinforced by forged rings. The bent blades are welded into position between the impeller discs. The blades are protected by screwed-on wear plates. These are numbered from 1 to 11 to ensure mounting of replacement plates. The plates are screwed on according to the number order. Tightening torque for the screws : 245 Nm. The shaft is of hollow design. The fan shaft has been rated so that the max. operating speed is below the critical speed. Impeller and shaft are connected by means of a flange. This screwing is protected by wear plates. The fan housing is sealed at the shaft passage to the outside by means of two-part labyrinth seals. Bearings The rotor is placed between oil-lubricated sleeve bearings. The drive-side bearing is designed as thrust bearing which absorbs the axial thrust of the rotor. The bearing housing is sealed towards the outside at the shaft passage by means of auxiliary seal kit. The bearings are lubricated with oil Thermometers are fitted to monitor the bearings temperature. Shaft sealing The shaft seals are fitted to the bearing pedestals and connected with the box section scroll by means of flexible coverings. The individual labyrinth sealing rings and the distance rings are held together by screws in the sealing casing. Regulation The fan is adapted to changing operating conditions bymeans of varying the speed of fan and also by adjustable inlet dampers arranged in front of impeller on either side. According to the required capacity, the speed of the fan can be varied and / or the inlet dampers position can be adjusted. For achieving speed charges a hydraulic coupling is provided Variable speed Turbo Coupling The turbo coupling is an infinitely variable fluid coupling with plain bearings and silumin rotating parts.

KORBA SIMULATOR

102

The oil pump is flanged on below the housing of the turbo coupling. During operation the filling pump in the housing delivers the required quantity of working oil and lube oil. An auxiliary lube pump also installed in the housing, ensures that lube oil side delivered when the couplings starts up or run down. As standby, another aux lub pump is installed. The primary runner, comprising primary shaft, primary wheel and shell is supported in the bearing housing and the coupling housing. The secondary runner comprising secondary shaft and the secondary wheel is supported in the scoop tube housing. AIR PREHEATERS STEAM COIL AIR PREHEATER Supplier : M/S Patels air temp. Pvt Ltd.,

Ahmedabad

Nos : 2/Boiler

Size of Steam Coil Air preheaters

: 4468 x 2990 x 336

Installed position : Horizontal Duct

Design Pressure : 16.5 kg/cm2g

Hydraulic Test Pressure : 25 kg/cm2 kg

Design temperature : 230OC

Weight of Steam Coil air heater : 1100 kg

Type : Modular Design type

The duty of steam air heater is to maintain the primary and secondary air heater average combined gas outlet and air inlet temperatures at pre-set values. The system description of one these circuits are given below. To achieve average primary air heater gas outlet and air inlet temperatures, the quantity of steam entering the steam air heater is regulated by a temperature control valve. For isolation purposes, four manually operated isolating valves along with steam traps are provided at inlet and outlet of each SCAPH. Condensate leaving the SCAPH passes through a isolating valve before entering the SCAPH drain vessel.

KORBA SIMULATOR

103

PRIMARY AIR PREHEATER Type: : Ljungstrom Bisector

Air Heater size : 27 VI (M) 80"

Rotor Drive Motor : 11KW GEC, 1450rpm, 415V, 3

phase, 50 Hz

Speed reducer : 5, APC (APCO) 110:1

App. oil Capacity : 50 Litres

Coupling : 11.5 FCU (Fluid Coupling)

Bushings : Taper lock Bush & Adopter

No. per Boiler : 2

Auxiliary Drive

Air Motor : R.S.M. 400 (APCO) 1" x 1" NPT

CHICAGO Pneumatic Air Motor.

Coupling : Bibby Coupling 124-A

Rotor Support Bearing: : SKF Spherical Roller thrust

Bearing SKF - 294/500

Rotor Guide Brg : 23060 Spherical Roller

RTD : Nether land Thermo Electric 1"

NPT

Oil Capacity of support brg housing : 150 litrses

Oil Capacity of guide brg housing : 20 litres

Oil Circulating System (Support BRG.)

Motor (TEFC) : 0.75 KW, 1500 RPM, Frame 80

flange cum 415 V, 50 Hz, 3 Phase

foot mounted GEC.

Pump: : Delaval - 1" x 1" NPT (APCO)

Filter : John Fowler 1" x 1" BSP

Cooler : Universal Heat Exchanger

Coupling : Lovejoy L – 095

Oil Circulating System Guide BRG.

Motor (TEFC)

: 0.55 KW, 1000 RPM, 80 lange cum

foot mounted 415 V 50 Hz, 3 phase

GEC.

KORBA SIMULATOR

104

Pump : Delaval - 1" x 1" NPT (APCO)

Filter : John Fowler 1" BSP

Cooler : Universal Heat Exchanger

Coupling : Lovejoy L - 095

Cleaning Devices

Motor (TEFC) : 018 kW, 1450 RPM, GEC - 71, 415

V 50 Hz, 3 phase

Reducer : All Royed 4900:1, 1 3/4

Coupling : Lovejoy flexible coupling L-075 &

L-110

SECONDARY AIR PREHEATER

Nos. per Boiler : Two

Heater size : 30.5 VI (M) 62" (68")

Rotor Drive :

Motor TEFC : 15 KW, GEC, 180 M, 1400 RPM,

415 V, 3 PHASE, 50 Hz, 29.5 A (FL)

Speed Reducer : 7 AP, Speed reducer (APCO) 130,

36:1

Oil capacity : 98 Litres (approx.)

Couplings : 11.5 FCU (Fluid coupling)

Bushing : Worthingdon Hub

Filter Lubricator : Velgan 256 Series 1" BSP

Auxiliary Drive

Air Motor : R.S.M. 270 (APCO) Chicaco

Pneaumatic Air Morot

Coupling : Bibby Coupling - 124 C

Roto Support brg. : 294/710 Spherical roller thurst

brg.

RTD : 1" NPT Nether land Thermo Electro

Oil capacity of support brg housing : 250 litres

Oil capacity of Guide brg. housing : 25 litres

Oil Circulating System (Support BRG.)

Motor TEFC : 0.75 KW, GEC, 1450 RPM, 415 V,

KORBA SIMULATOR

105

50 Hz, 3 ph

Pump : DELAVAL (APCO) 1" x 1" NTP

FIlter : John fowler 1" x 1" BSP

Cooler : Universal Heat Exchanger

Coupling : Lovejoy, L-095

Cleaning Devices

Motor TEFC : 0.18 MW, NGEF, 750 RPM,

Coupling : Lovejoy - L - 075 & L - 110

PRIMARY AND SECONDARY AIRHEATERS The Rotary Regenerative Air Preheaters are designed for use on plant where hot air is required for combustion or for fuel saving. The Air Preheaters contain in a small space, heating elements of a large surface area. In regenerative heat exchangers, the heat transfer surface is alternately heated by the flue gases passing through it and cooled by the air passing through it. The flue gases and the air flow through the same passages at different times so that unlike the recuperative heat exchanger where heat flows through the passage walls from the flue gases to the air, the heat is absorbed by the regenerative mass from the hot flue gases and then released to the cold air. This process can be periodic or if the regenerative mass rotates, as in this project, the process is continuous. In the Air Preheaters, flue gas flows through one side of the regenerator and through the other side flows the incoming air prior to entering the furnace. The regenerator is slowly revolved so that the heating elements pass alternately through the steam of the hot flue gases and through the stream of cold air. A portion of the heat in the flue gas side is transferred to the air when the elements pass through the airside, so heating the flow of air and thereby cooling the elements. Thus the heat in the flue gases is partly recovered and returned to the furnace via the airflow. The two streams, flue gas and air, which flow through diametrically opposite segments of the rotor, are separated form each other by a small blanking section with sealing plates to form a division between them. The two streams flow in opposite directions, i.e. in contra flow. In this particular plant the flow arrangement is gas down and air up. Basic Construction The rotor is the central part containing the heat transfer matrix. The rotor is radically divided into twelve sectors. The heating elements are arranged in these twelve sectors in two or more layers. The housing surrounding the rotor is provided with duct

KORBA SIMULATOR

106

connections at both ends, and is adequately sealed by radial and circumferential sealing members - forming an air passage through one half of the preheater and a gas passage through the other. The weight of the rotor is carried on the underside by a spherical roller thrust bearing whilst at the top a spherical roller guide bearing is provided to resist radial loads. The rotor revolves continuously absorbing heat from the flue gases and transferring it to the air for combustion. Each airheater is provided with a electric motor drive for normal operation and an air motor drive for emergency and also for use during off load water washing. Rotor Seals Seals are provided at both ends of the airheater to minimise leakage between the airside and the gas side of the preheater. The hot and cold end radial seals are attached to each diaphragm of the rotor and are set at a specified clearance from the sector plates which separate the air and gas streams. The hot end sector plates are automatically deflectable to provide leakage area reduction during transient as well as full load operation. The seals provided at rotor post are set to operate with minimum clearance with respect to the horizontal sealing surface of the sector plate centre section. The bypass seals provide sealing between the periphery of the rotor and sealing surface of the connecting plate and/or the preheater housing. Axial seals are provided vertically in the rotor shell in line with radial seals. Heating Surface Elements The heating surface elements in the cold end are manufactured from thin steel sheet adjacently, one being undulated and the other being thin sheet steel. The notches run parallel to the rotor axis and space the plates the correct distance apart. As the cold end, i.e. gas leaving - air entering end of the preheater, is most susceptible to corrosion due to temperature and fuel conditions, the elements are arranged in tiers. The lower or cold end tier of elements is manufactured from corten steel to combat corrosion and is termed “cold end elements”. The middle tier termed the “hot end elements” and are both made from carbon steel. All elements are packed into containers to facilitate removal and handling. The cold end packs are arranged such that they can be withdrawn from the rotor in a radial direction without disturbing the hot end and intermediate packs.

KORBA SIMULATOR

107

The ‘Hot’ and ‘Intermediate’ ends are provided with double undulated type heating elements. The undulations provide high turbulance to the gases and air passing through the preheater. Rotor Drive Assembly The driving force for turning the rotor is applied at its periphery. A pinion attached to the low speed shaft of a power driven speed reducer engages a pin rack mounted on the rotor shell. An air motor is provided as an auxiliary drive for the airheater. This drive ensures the continued operation of the preheater, even if power to the electric motor is interrupted. The air motor may also be used to control the speed of the rotor during water washing of the heating surfaces. Rotor Bearings A spherical roller thrust bearing supports the complete rotor. The load is transmitted to the thrust bearing by a trunnion, bolted to the lower end of the rotor post. To guide the upper end of the rotor a guide trunnion is bolted to the face of the rotor. Oil Circulating Systems Separate oil circulating systems are provided to supply support bearing and guide bearing with a bath of continuously cleaned oil at the proper viscosity. The bearing oil supply is circulated by means of a motor driven pump through an external filtering systems. A thermostat is used to limit the operation of the system to temperatures, which will ensure against overloading the pump or motor as a result of high oil viscosities. Soot Blowers Both primary and secondary airpreheaters are provided with twin nozzle swivelling arm type electric driven soot blowers for onload cleaning at gas outlet end only. Water washing and Fire fighting Two fixed multi-nozzle washing manifolds are fitted, one the hot end, the other on the cold end for off load water washing of airpreheaters. A deluge system, incorporating headers with special nozzles strategically located in the hot end and cold ends of the airpreheater is provided for use in case of a fire inside the airheater.

KORBA SIMULATOR

108

Rotor Stop Alarm The airheater rotor should not be stopped when high temperatures gases are flowing through it. The rotor stop alarm system is provided to give immediate warning that the rotor has stopped so that action can be taken to prevent damage occurring from overheating. Fire Detection Equipment The fire detection equipment is provided to detect any hot spot in the airpreheater rotor during operations. The system consists of a sensing head; drive system to drive the scanning head across heating elements, compressed air provision for cleaning the head, and cooling water system for the sensing head. Two numbers of sensing heads are provided for primary and four Nos. of sensing heads are provided for secondary airheaters. These are located in the respective air inlet ducts. ELECTROSTATIC PRECIPITATOR Design condition

Gas flow rate : 980 M3/sec

Temperature : 140OC

Dust Concentration : 73.5 gm/NM3

Type : FAA - 7 x 36 - 4 x

48125 -2

No. of Precipitator offered per boiler : Four

No. of Gas path per boiler : Four

No. of fields in series in each gas path : Seven

Pressure drop across the precipitator for design condition : 18 MMWC

Velocity of gas at electrode zone on total area : 1.0 M/Sec

Treatment time : 25.2 Sec

Collecting Electrodes

No. of rows of colllecting Electrods per field (9 plates are

arranged in each row)

: 65

Total No. of Collecting plate per boiler : 16380

KORBA SIMULATOR

109

Nominal height of Collecting plate : 12.5 Meter

Nominal length of collecting plate : 400 MM

Specified collecting area : 164.57 M2/M3/sec

Emitting Electrodes

Type : Spiral with hooks.

Size : 2.7 MM Dia

No. electrodes in the frame forming one row : 54

No. of electrodes in each field : 3456

Total No. of electrodes per boiler. : 96768

Total Length of electrodes per boiler. : 502226

Plate wire spacing : 150 mm

Rectifier :

Rating : 70 kV (Peak) 800 MA

No of rectifier per Boiler : 56

Type : Silicon Dicde, Full

wave bridge connection

Location : Mounted on the top of

precipitator.

Motor for Rapping Emitting Electrodes

Qty : 112 Nos.

Rating : Geared Motor 0.33

H.P./2.5 RPM, 3

phase, 415 V, 50 Hz.

Motor for Rapping of Collecting Electrodes

Qty. : 28 Nos.

Rating Geared Motor 0.5

H.P./1/1/2/5 RPM, 3

phase, 415 V, 50 Hz

Rappers For Collecting Electrodes

No. and type of Rappers : One drop Rammer per

row of collecting

electrodes having a

KORBA SIMULATOR

110

collecting surface of

90.0 M2.

Rapper Size : 4.9 Kgs

Frequency of Rap. : Varying from 10

raps/hr at the inlet

field to 1 rap/hr at the

exit field. The

frequency of rapping

for the intermediate

field can be adjusted

between 10 and 1 per

hour according to

requirements.

Drive : Geared electric Motor

Controlled by

synchronous

programmer.

Location : On the bottom of side

panel of EP casing.

Rappers for Emitting electrodes

No. of type of rappers : One drop hammer per

two rows of Electrodes

Rapper Size : 3.0 Kgs.

Frequency of Rap : 10 raps/hr

Drive : Geared Electric Motor

Controlled by

synchronous

programmer

Location : On the top panel of

E.P. Casing

Rapper for Gas Distribution System

Qty. : 4

Rating : Geared Motor 0.5 HP

KORBA SIMULATOR

111

1.1 RPM at 3 Phase

415 V, 50 Hz.

Location : On the G.D. Housing

side panels of the

casing.

The gas cleaning plant consists of four BHEL make Electrostatic Precipitators type 4xFAA-7x36-4x48125-2. The units are designed to operate on the exhaust gases from each of the 500 MW Steam Generators. The exhaust gases to be treated pass along the inlet duct and enter the steel precipitator casing via an inlet funnel. To ensure the gases are evenly distributed across the full sectional area of the treatment zones, splitter plates within the inlet funnel and two rows of distribution screen at the inlet of stream are positioned.

After treatment by successive zones within the precipitators the clean gases pass through the outlet funnel and flow along the outlet ductwork connected to the I.D. fans and are hence discharged to atmosphere via the chimney. In order to maintain the required standard of gas distribution within the precipitator, vertical outlet baffles are located immediately after the final treatment zone. Each precipitator is designed for two horizontal streams of gas flow. Each stream is having six treatment zones or fields. Each treatment zone consists of parallel rows of sheet type collecting electrodes suspended from the precipitator casing with wire type discharge electrodes arranged mid way between them, fixed to upper and lower frame assemblies. Each separate electrical zone, comprising of the discharge electrodes, is suspended from a discharge suspension arrangement mounted on the casing top plate. The transformer-rectifier sets, one per zone - seven total per stream, are arranged at top house access platform level adjacent to each relative zone. The respective control panels and L-T distribution equipment’s are located within the control room built at ground level immediate to each precipitator. Seven rapping gear motors for collecting electrodes and fourteen rapping gear motors for emitting electrodes are provided for each stream. The rapping gear operates continuously to dislodge the precipitated dust, which falls under gravity into the pyramid type hoppers, located directly beneath each treatment zone, for removal by the ash handling system. To assist the dust to remain in a free flowing state, electric heaters are provided externally at the bottom portion of each hopper.

KORBA SIMULATOR

112

Each pair of precipitators is served by an arrangement of access platforms and stairways from ground level for the top housing level. To facilitate removal and replacement of the transformer rectifier sets and other maintenance, a lifting beam arrangement is provided at the top house roof level on each casing of precipitator. A single hoist and geared trolley is provided to servo each lifting beam arrangement. For the safe operation of these precipitators a full safety interlock system is provided. SOOT BLOWING SYSTEM Introduction On load, gas side cleaning of boiler tubes and regenerative airheaters is achieved using 126 microprocessor controlled sootblowers which are disposed around the plant as follows :

1. 88 -Furnace Wall Blowers – Steam 2. 34 - Long Retractable Soot Blowers - Steam 3. 4 - Airheater Soot Blowers for Primary and Secondary Airheaters – Steam

The boiler waterwall panels are provided with suitable wall boxes four for future accommodation of an extra sixteen furnace wall blowers and twenty-four long retractable sootblowers for upper furnace, arch and rear pass zone, if necessary. Soot Blower Piping Steam for sootblowing is taken from the division panel outlet header. To sootblow the regenerative airpreheaters during boiler start-up, however, a separate connection is also provided from the auxiliary steam system. The supply pipework from superheater of steam source (division panel outlet header) is fitted with a hand operated and motor operated isolating valves followed by a pressure control valve and a spring loaded safety valve as protection against steam over pressure. the safety valve vents via expansion chamber closed at determined by the operator in relation to boiler load via the sootblower control system. From the steam source after the pressure reduction the main line is split into six sootblowing sections.

KORBA SIMULATOR

113

KORBA SIMULATOR

114

Steam is fed through various Sections at the steam main pressure of 30 kg/cm2 (g). Further reductions to the blowing pressure are achieved by adjusting the setscrews of the individual soot blower valve heat at the time of soot blows system commissioning.

Branches of the section pipelines supply steam to individual soot blowers. At various points on each section, pipeline connections are made via motor operated drain valves to the intermittent blowdown tank. These drain valves are all operated by the sootblower control system. On request to soot blow, the control system will open the sections drain valves and crack open the inlet steam main isolating valve in order to warm up the soot blower pipings and drain any condensate to the intermittent blowdown tank. Once the piping is proved to be warmed up, resulting in no condensate being produced, the control system will close the drain valve and fully open the inlet-isolating valve thus bringing the pressure control valve into operating and signally commencement of soot blowing sequence operation. The temperature control valves/drain valves automatically as per the setting maintain steam temperature in each section. SOOT BLOWER

Source of steam for soot blowing Tap off after divisional panel

Set pressure on pressure reducing valve 30 Kg/cm2

Set pressure on safety valve 39.5 Kg/cm2 (g)

Maximum flow rate 21000 Kg/hr

Steam pressure for soot flowing 26 Kg/cm2

TYPE OF BLOWER

WB IE LRD II E LRD II E LRD II E

Blower Number 1 to 88 105 & 106 107 to 120 121 to 138

Travel in MM 305 9200 9200 9200

Dead Travel in MM - 350 350 350

Nozzle in MM 26 32 25 25

Blowing pr, in Kg/cm2

(g)(2 Blowers) in Min.

12 12.5 12 9

Operating time per

group(2 Blowers) in

Min.

1.5 11.87 11.87 11.87

KORBA SIMULATOR

115

Operating time per

Cycle (All. Blowers) in

Min.

66 11.87 83.09 106.83

Blowing rate per group

in Kg/Minutes

83.3 293.33 176.66 143.3

Consumption per group

in Kg.

116.62 3349.8 2017.45 1636.48

Steam consumption per

cycle in Kg.

5131 3349.8 14122 14728

WBIE: : Wall De Slagger

Electrically operated

LRD II E : Long Retractable Soot

blower Electrically

operated

Steam temp. for Soot Blowing

: 250oC

Steam Constumption rate per group of wall Blower : 500 Kg/hr

Type of wall blower : RW 5E

Wall Blowers The blower assembly consist of a stationary body and rack gear housing and a rotary gearbox assembly to which the swivel tube assembly is attached. The swivel tube assembly is supported by bushings at each end of the body casting. The horizontal guide rods are used to assure proper alignment of the rotary gearbox assembly. A stationary electric motor is situated on the right side of the blower. This motor, through a rack gear housing assembly operates a pinion, which drives a horizontal rock assembly, the outer end of which is fastened to the rotary gearbox assembly. When the rotary gearbox approaches the fully extended position a ramp cam attached to the free end of the rack contacts a bearing surface, which is a part of the clevis bracket assembly and bushes the valve stream assembly addmitting steam to the swivel tube. When the blower is started the rack pinion moves the rack and rotary gearbox towards the boiler. Operation of the rack gear housing causes rotation of a shaft extending out from the rack gear housing into a switch box. Located in this switch box are two cam actuated limit switches. One can holds limit switch LSTE in

KORBA SIMULATOR

116

open position when the blower is fully retracted. Extending of the blower moves the cam allowing LSTE to close. the blower is then under its own control. Near the fully extended position, the ramp cam strikes the lever that opens the SBV head valve. The second limit switch cam strikes the LSTS limit switch, which opens the circuit to the traversae motor and closes the circuit to the rotary motor. The rotary motor is attached to the gearbox assembly. When LSTS closes, the motor rotates the swivel tube through a gear train. When the blowing sweep is finished, the cam assembly on the swivel tube contacts and rotates the arm on the limit switch LSTR. The traverse motor begins to retract the blower. Near the fully retracted position the cam again opens the switch LSTE to halt the blower. Long Retractable Sootblower The LRD -IIE model Soot Blower is a boiler cleaning device in which a rotating lance extends into and tetracts from the boiler to make sure that the cleaning medium -steam-directed through the nozzles, removes the deposits form tube surfaces. The lance is attached to a carriage housing, which runs on tracks inside the blower housing. The carriage and lance are moved by means of a traversing chain operated by a electric power pack. Rotary motion is applied to the lance through the travelling carriage by a second chain driven by a separate electric power pack. Control movement is by a stop limit switch and a reverse limit switch. The unit can be supplied with different traversing and rotating speeds. Standard traversing speeds are available in various increments from 1.25 to 3.65 m. per minute. Standard rotating speeds are available in various increments from 4.25 rpm to 7.75 rpm. These speed variations are accomplished by changing the power pack and jackshaft drive sprockets. Other speeds are possible for special application by the use of special sprockets. Flow of blowing medium though the retractable soot blower is controlled by the valve mounted at the rear end of the blower. The feed pipe is attached to the outlet of this valve head. This feed pipe passes through packing gland in the travelling carriage and lies inside the lance tube extending to almost the entire length of the blower. The wheels on the travelling carriage run on tracks welded to the inside of the blower housing. A roller on each side of the carriage limits sideways motion, which use the housing sides as guides. The ends of traversing chain are connected to each end of the carriage. The rotary chain is continuous. It passes over sprockets on the carriage and causes rotation through a gear train.

KORBA SIMULATOR

117

The lance is flanged to the carriage and supported on the boiler end by bearing and yoke plate. The electric gearbox on the right side is for traversing and the electric gear box on the left side is for rotation when viewed from the rear end of the blower. Motion is transmitted from the gearboxes to jack shafts on each side of the blower. Tension on the internal chains is adjusted by adjusting the screws on chain tighteners, which hold the idler sprockets on the outboard end of the unit. The housing completely covers the blower except the traversing and rotating gearboxes. The housing is open at the bottom except for tie bars at intervals. A section of the top of the housing near the rear end of the blower is cut away to allow access to the travelling carriage. The access areas have removable cover. A shot section of the track at the rear is removable to permit removing the travelling carriage for major maintenance. The soot blow valve head is operated by a trip pin on the top of the travelling carriage which engage a trip cam and through the trip rod linkage and valve lever causes the head valve to open or closes. The length of the trip rod governs the stroke of the head valve. To change the valve stroke, loosen the join nuts where it screws into the rod connection and turn the rod. One end of the rod has a right hand thread, the other end in left hand. When the desired length is attained tighten the join nuts. The spring on the trip rod should be adjusted to eliminate all looseness in the assembly. Airheater Sootblower The cleaning device consists of an electric motor coupled to a gear driven crank mechanism, which oscillates the swivel header carrying the twin nozzle pipes. The cleaning medium is conveyed through the swivel heads and respective nozzle pipe, to the nozzle at the end. A rotary point in the supply line permits free motion of the swivel header while connected to the source of supply. The arc traversed by the nozzle and the rotation of the rotor subject the entire area of the rotor to the action of the cleaning jet.

KORBA SIMULATOR

118

To maintain the desired steam pressure at nozzle an orifice plate is provided in the supply line. Drain connections are provided in the steam piping at suitable locations for removing condensate from the piping system while the device is idle, and just before it is placed in operation. The steam supply line to cleaning device has to be supported in such a way to avoid axial and side thrust being applied on to the rotary swivel joint. If care is not taken in this regard, heavy leak may occur in this joint. DA head valve is provided to reduce the inlet steam pressure to the required blowing pressure. PROGRAMMABLE SOOTBLOWER CONTROLLER Introduction The Programmable Soot Blower Controller, herein after referred to as the PSC, is essentially a mini computer programmed to carry out system logic decisions. The logic decisions are accomplished through a set of digital instructions, called the executive software, stored in the memory of the minicomputer. All permissive limits and interlocks are programmed into the executive software together with other system parameters, such as the number and types of soot blowers programmed to operate upon each program step are placed under software control into the memory system of the PSC by means of the program panel. The PSC is divided into four functional parts: the control panel, consisting of display and operator switch inputs; the controller input/output cabinet, containing the output drivers, receiving circuits and computer components, a program panel which is connected whenever a program change is desired; and other optional peripheral equipment as dictated by job application, such as data logging, analog to digital conversion equipment, and alarm annunciation. The PSC is capable of opening all soot blowers in the system, constrained within limits permitted by the executive software. It has an expansion capability of up to 510 soot blowers. With its emergency backup control system, complete display capability and remote manual soot blower operation is possible should the computer become disabled. These features and more provide a modern, versatile and reliable controller needed for efficient boiler cleaning. Component Description Display and Control Cabinet The display and Control cabinet, normally located in the control room for use by the PSC operators contains the panels for graphic display and for control of the soot blowers.

KORBA SIMULATOR

119

The graphic display panel, located in the half of the cabinet, shows individual soot blower operating status and the following general system indicators :

• Control Power Failure • Controller Operating • Low Header Pressure • Motor Overload • Soot Blower in Service • Soot Blower Blowing • No Blowing Medium • Blower Start Failure • Time Exceeded

And others as required by the job. The control switch panel, located in the lower half of the cabinet, contains the control pushbuttons. The functions of these controls are as follows:

• Program Selection • Program Start • Cascade Start • Program Stop • Program Reset • Program Cascade • Program Operating Display • Program Check • Sequence Check • Error Acknowledgement • Soot Blower Manual Start • Soot Blower Retract • Soot Blower Status Check • Soot Blower Enable/Disable

and others as required by the job. Normally, the control switch panel is separated into two parts; one for independent control of the retracts, and the other for independent control of the wall blowers. Both panels open from the front to reveal the interior circuitry and hardware. Located on the control switch panel cabinet are the emergency override key switch, power supplies, power supply monitoring meters, and program panel socket. Behind the graphics panel are contained the required unit light driver circuitry. The graphics display consists of a multicoloured film negative through which the light from replaceable L.E.D.S is illuminated. A light driver located on light driver cards drives each of the L.E.D.s. Each light driver card has the capability of energizing sixteen

KORBA SIMULATOR

120

L.E.D.s. The card cage in which the light driver cards are located has the capacity for sixteen light driver cards; providing a total capability of 256 light drivers with each card cage. Each card cage contains one display decoder control card. The function of this card is to decode the unique blower address signal and energise the light driver for that blower. The control panel contains the necessary, push-button switches for starting, stopping, cascading, and resetting soot blowing programs; enabling, disabling, and starting of individual blowers; verification and status checks for both programs and blowers; and the Emergency Override keyswitch and Program Panel socket. The control switches are mounted to printed circuit cards located immediately behind the front panel. The cards condition the signals for input to the computer. Inside the display cabinet are the two power supplies required by the control and display panel plus and interface card cage that contains an array of multiplexing modules for sharing of the switch control signals. Also located on the card cage is power supply metering, and the multiplex control card, designated the DMC card. This card processes the graphics control information, energizes the switch multiplex array, and serves as the emergency control centre in the emergency override mode. Soot Blower Program Panel The Soot Blower Program Panel (or box) is used to create programs (soot blowing routines). Connected to the display and control panel via an umbilical card, the Program Panel in conjunction with the graphic display allows the PSC operator to set up the various programs.

KORBA SIMULATOR

121

CONDENSATE AND

FEED WATER SYSTEM

KORBA SIMULATOR

122

KORBA SIMULATOR

123

CONDENSATE EXTRACTION PUMP SPECIFICATIONS

Type of first impeller : Double Suction

Number of stages : 5

Design flow rate : 8,10,00 Kg/hr

Inlet – Temperature : 43.1OC

Inlet - Specific Gravity : 0.991

NPSH required : 4.1 meter

Discharge pressure : 30.4 ata

Total Dynamic Head : 307 meters

Shut off Head : 398 meters

Pump Speed : 1480 rpm.

Power Consumption at Design Condition

Power input at the motor : 899.2 KW

Losses in the motor : 41.4 KW

Power input to the pump : 857.8 KW

Efficiency of the pump alone : 79.0%

Overall efficiency of the pump motor : 75.37%

Guaranteed power consumption with a flow rate of

605,000 kgs/hr.

: 776 KW.

Sealing Water requirement

Flow : 10 litre/min

Pressure : 2-4 ata

Temperature : 30-50OC

Minimum flow for continuous stable operation : 350 T/hr

Suction stainer size : 350 microns

Radial Bearings lubrication with Water : 4 Nos.

Motor Specification

Output : 1120 KW

Voltage : 6.6 KV

Number of poles : 4

Speed : 1480 rpm

Cooling water Requirement for one motor

KORBA SIMULATOR

124

Motor upper Bearing-Thrust Bearing : Oil lubricated

Flow : 30 litres/min

Pressure : 2.0 to 7.0 Kg/cm2

Max. Inlet temp. : 40OC

Motor inner bearing lubricated with : Grease

Coupling : Rigid Type

Rating : 1500 HP

Size : 360 mm

Condenser

Type : Twinnest, Double pass,

Single shell

Area of cooling surface : 3253 M2

Number of xooling tubes : 24710

Lenth of each tube : 14730 mm

Size of tube-(OD x thickness in mm) : 28.575 x 0.7112

Tube material - Stainless Steel tubes : SS.304

Weight of empty condenser : 640 MT

Weight of tubes : 180 MT

C.W. flow : 55017 m3/hr

C.W. velocity : 2.13 M/s

C.W. design temperature : 28OC

C.W. Max. temperature : 35OC

Back pressure at MCR : 60.45 mm of

Temperature rise max. : 10.56OC

Terminal temperature Difference : 3.17OC

Head loss on C.W. side : 6.3 MWC

Fouling factor : 0.9

Condenser tube cleaning system

Name of the supplier : TAPROGGE

No. of Cleaning systems : 2

No. of Balls : 800 Nos.

Strainer Section

KORBA SIMULATOR

125

Quantity per TCS : 2 Nos.

Slope of Screen : 30OC

Width of Gap : 10 mm

Cooling water : 7.639 kg/s

Actuation : Motor operated

Ball recirculating pump

No. : 1 per unit

Type : KWPK-80-250

Normal capacity : 45 m3/hr

Normal discharge pressure : 1.8 bar

Pump motor : Siemens

Operating voltage : 415V/50 Hz

Rated output : 5.5 KW

Full load speed : 24

CONDENSATE EXTRACTION PUMP - DESCRIPTION Each condensate extraction pump which is driven by a 1120 KW induction motor, delivers 810,000 kg/hr of condensate water against 307m. Of total dynamic head at the rated condition. CONSTRUCTION OF C.E. PUMPS The pumps are of the direct driven by a constant speed motor through a rigid coupling, vertical barrel, double suction, multi stage, diffuser type. The pump consists of internal assembly, discharge assembly and suction barrel. The internal assembly comprises of 5 stage casing, a guide vane, five impellers, 2 column pipes, a suction bell and shaft and is submerged in water in the suction barrel. The discharge assembly comprises a discharge head with a stuffing box to seal the pump shaft and is installed on the suction barrel. The suction barrel is installed on the pump floor. Water is admitted into the casing from the suction barrel through the suction bell and first stage casing and discharged through the column pipes by the energy imparted by the impellers.

KORBA SIMULATOR

126

CO

ND

EN

SA

TE

SY

ST

EM

KORBA SIMULATOR

127

Internal bearings (Leaded bronze bearings) installed in a column pipe and the top casing are provided for supporting the pump shaft against the radial load. Upper and lower bearings (leaded bronze) are installed in the stuffing box and suction bell. The weight of the pump rotor and the hydraulic thrust acting on the rotor in the axial directions are supported by the thrust bearing in the motor. The impellers are driven by a 1120 KW vertical shaft induction motor mounted on the discharge head. The coupling spacer is furnished between the pump and motor in order to remove the gland and seal ring seal without removing the motor. Adjustment nut is provided at the top of the drive shaft to facilitate adjustment of the axial location of the rotating part. Gland packings are used for shaft sealing. Start up Checks Check that there is no foreign material in pump. Check that the condenser is cleaned up. If the condenser is dirty, suction strainer of pump will be clogged frequently. Suction strainer shall be kept installed during initial operation and remove it after system gets cleaned. Open suction valve fully and fill pump with water. In this process air vent valves shall be fully opened to purge air completely.

Ensure that the following valves are opened.

1. Suction valve

2. Sealing water inlet valve

3. Iso. Valves of respective coolers

4. Balance valve.

5. Pump’s Recirculation valve.

Keep the pump’s disch valve closed. Check the water level in the condenser is adequate. Check that lub. oil is filled up to the mark. Turn on circuit breakers of respective equipment’s and auxiliary devices.

KORBA SIMULATOR

128

Once the pump is started and reaches full speed, trip the pump. Check that there is no abnormal noise etc. Restart the pump Check the condition of gland leakage. It should be just in continuous drips. CHECKS DURING OPERATION MECHANICAL

1. Do not operate the pump with the discharge valve closed for more than a few minutes.

2. Sufficient care shall be taken for abnormal noises.

3. Observe bearing temperatures, vibrations, discharge pressure etc.

4. Ascertain that all indicators show proper value under pump running condition.

5. Gland sealing water pressure 1 to 3 kg/cm2

6. Pump bearing temperature Max. 80OC

ELECTRICAL

1. Motor input power shall be checked.

2. Over load conditions shall be checked which will badly effect motor service life.

3. Power source voltage fluctuation shall be checked.

4. Over current shall be carefully checked.

5. Motor winding temperature to be within limits.

SHUT DOWN When the pump is shut down for standby duty, care shall be taken on the following :

1. Switch off the motor

2. Pump discharge valve will be completely closed and selected to auto.

3. Space heaters should remain switched on.

KORBA SIMULATOR

129

LONG PERIOD SHUT DOWN

1. When the pump is not used for more than one month, operate the pump for approximately 30 minutes, to keep the equipment in good condition.

2. Cooling water system should remain isolated.

3. Space heaters should be kept on.

ISOLATION Isolate the motor electrically. Isolate the cooling water system and drain it. Pump Discharge and suction valves, balance valve, will be closed as the pump will be drained.

Pump’s sealing water system will be isolated.

CONDENSATE SYSTEM DESCRIPTION OF SYSTEM The purpose of this system is to store an adequate quantity of demineralized water to meet the make-up requirements for normal cycle fluctuations and for abnormal operating conditions when supply of demineralized water is interrupted. In addition, this system will transfer condensate to and from storage tanks as needed to satisfy main cycle requirements. The main cycle flow and thermodynamic requirement is maintained by transporting the condensate collected in the condenser hotwell through various stages of feedwater heating and other equipment to the deaerating feedwater heater. The condensate extraction pumps normally deliver the condensate through the three low pressure feedwater heaters, the deaerating feedwater heater to the deaerating storage tank, which is the beginning of the feedwater system. The low pressure feedwater heaters receive extraction steam from the turbine. The condensate absorbs heat from the extraction steam as it passes through the feedwater heater. The deaerating feedwater heater further preheats the condensate prior to its entry into the deaerating storage tank. The condensate in the deaerating feedwater heater is warmed by extraction steam during normal operation and auxiliary steam & cold reheat steam are utilised as the heat source during start-up & turbine shut down condition. The normal make up to the condenser is supplied from the demineralizing plant through the make up pumps. In case of fluctuations in the cycle, condensate will be transferred to and from the condensate storage tank as required. Normally, on low

KORBA SIMULATOR

130

level in the condenser hotwell, condensate will flow from condensate storage tank to hotwell by static head in the tank and differential pressure due to condenser vacuum, however, should this flow be inadequate, the condensate, transfer pump will supplement the flow. This make up is sprayed into the steam space above the tube bundles. The condenser hotwell is condensate collection vessel, integral with the condenser shell, and located in a pit below the ground floor. Condensate collected in the hotwell is pumped by 3 x 50 % Condensate Extraction Pumps to the feed storage tank through feedwater heaters placed in series. Two lines from hotwell, make a common header where from three lines are connected at the suction of three Condensate Extraction pumps. A strainer is placed at the suction of each condensate Extraction pump to collect debris during commissioning. The suction piping to the pumps is vented back to the condenser, to insure that the non-operating pump(s) stays completely flooded. These vent lines include manual valves on the vent for each pump. A minimum flow (350 T/Hr) recirculation line for each pump is provided, returning to the condenser via a flow control valve and a locked open shut-off valve. The shaft seals of these pumps are the water-injected type fed from a header to prevent the suction of air, particularly the pump that is not operating while the condenser is under vacuum. One discharge line emerges out from each condensate extraction pump with one check valve and one motor operated stop valve placed in series. These lines from a common discharge line and enters the turbine gland steam condenser. SYSTEM CONTROL Three 50 % Condensate Extraction Pumps shall be controlled from the Central Control Room (UCB). The Condensate Extraction Pumps are protected by safety interlocks to prevent eventualities like dry running, low NPSH and minimum flow conditions. The pumps are provided with Auto starting feature. A three position selector switch inscribed with ‘LEAD-NORMAL-LAG’ has been provided to select the pump for Auto Starting. The first pump to be started on ‘Auto’ shall be selected in ‘Lead’ position and the second stand by pump shall be selected in ‘LAG’ position. Any pump can be selected for Auto start either in ‘Lead’ or ‘Lag’ position. The pump on standby duty is streamlined to automatically start in the event of decreasing discharge header pressure below 30 ata (approx.) through’ 0-15 secs. delay or if the CEP disch. Header flow exceeds one pump capacity (810 T/Hr. Approx.) through’ 0.5-5 secs delay or a trip of the running pump. Pressure switches provides actuating signal for stand by pump to start. Each condensate extraction pump’s suction strainer is provided with differential pressure switch. These switches actuate control Room alarm in the event of high differential pressure (0.1 kg/cm2 approx.).

KORBA SIMULATOR

131

The condensate extraction pumps are also provided with level switches that monitor condenser hotwell level. The level switch protects the Condensate pumps from operating under very low (-1290 mm) suction head conditions. One level switch provides a pre-alarm for a low level (-1140 mm) condition. Low suction conditions are usually encountered during start-up or transient plant operations. The condenser hotwell is normally maintained by the operation of two level control valves. Normal level is maintained by level control valve LCV 0508 sensed by flow transmitter LT 0508. In the event this level control valve is unable to maintain normal level, the emergency make up control valve LCV 0509 comes into action, sensing low level in the hotwell by flow transmitter LT 0509. Normally the emergency make up will flow from condensate storage tank to hotwell by gravity. But even with above flow the hotwell level becomes low (sensed by level switch) then the condensate transfer pump shall start and its discharge valve shall open automatically. When the hotwell level is restored to normal level the above valve shall close automatically. Full closing of this valve would cause auto stopping or associated condensate transfer pump. Condensate spill control valve is provided in the line that connects the discharge header (after gland steam condenser) to condensate storage tank. This level control valve is automatically positioned by the hotwell level controller when a high hotwell level condition develops. Condensate is then transferred to the condensate storage tank until the hotwell level returns to normal. The condensate spill control valve is also provided with a motor operated. Bypass valve which can be operated manually from UCB in the event of controller malfunction. Each condensate extraction pump is equipped with individual recirculation control valve which ensures minimum flow through condensate pump when individual pump discharge flow falls below 350 T/Hr. sensed by flow transmitters. Beside above, to ensure minimum flow through the gland steam condenser, a minimum flow recirculation line and control valve is connected from the discharge header before Deaerator level control block valve back to condenser. During normal operation, condensate passes through the LP heaters 1,2 & 3. However, in the event of very high level in individual heater the condensate is automatically bypassed by interlock action. For this reason motor operated. bypass valves are placed across LP Heater respectively. In the event of very high water level in each LP Heater, sensed by level switches, the individual bypass valve opens and inlet-outlet motor operated isolation valves in each heater gets closed. Restoration of the heater level to normal will not automatically restore the heater to service. The return to normal operation must be initiated by operator action. A three element (Deaerator level, Deaerator input flow & feed flow), control loop is employed to maintain Deaerator level. Deaerator level is sensed by level transmitter A & B. This input is fed to a controller where signal from feed flow as well as Deaerator input flow i.e. the sum total of condensate and Heater drain flow is fed. Either of the two 100% flow control valve, position automatically to maintain the Deaerator level, receiving input from the controller.

KORBA SIMULATOR

132

In the event of high level in Deaerator, sensed but level switch, the level is restored by opening automatically the Deaerator high level drain valve. If the rise in level still persists and reaches very high level (+440 mm), sensed by level switches, then Deaerator level control block valves (on main condensate line) shall close automatically. On restoration of normal level above block valves have to be opened manually from remote. The drain valve shall close automatically when the Deaerator level falls below the high level. When conductivity at the outlet of each vessel of the condensate polishing unit becomes more than 0.1/us/cm, the unit regeneration shall be started by manual intervention.

KORBA SIMULATOR

133

BOILER FEED PUMP & AUXILIARIES Booster Pump for BFP

: For Motor Driven BFP

For Turbine Driven

BFP

Manufacturer : Weir Pumps Ltd Weir Pumps Ltd

Type : FATE 64 FATE 64

Direction of rotation

Anticlockwise(Viewed on

drive end)

: Anticlockwise

Anticlockwise

Thrust Bearing

Manufacturer : The Glacier Metal Co. Ltd The Glacier Metal

Co. Ltd

Type : Double thrust

Size : M8112/2P/2P

Mechanical Seal for Booseter Pump of BFP

Type : 4" Crane type 8B1. Spring loaded carbon face

pressing against a silicaon carbon seat.

Seal Pressure : 9.67 Bar

Temperature : 164.6oC

Shaft speed : 1494 rpm

Seal cooling : Closed loop recirculation via pumping ring through

a heat exchanger.

Operating Detail : Motor Driven Turbine Driven

Runout Design Runout Design

Specific gravity of Feed water

at suction temperature

: 0.901 0.918 0.900 0.918

Suction temperature oC : 164.6 148 164.6 148

Suction pressure bar : 9.67 7.2 8.17 8.17

Discharge pressure bar : 20.27 17.55 20.1 17.76

Differential pressure bar : 10.60 10.35 11.93 12.06

Differential head m : 120 115 135 134

NPSH above impeller eye-M : 30.5 30.5 13.5 13.5

Flow rate m3/hr : 1080.5 1242 1080.3 1242

KORBA SIMULATOR

134

Efficiency % : 80.5 82 79.5 31

Speed rpm : 1419 1494 1494 1515

Power KW : 395 436 450 514

Boiler Feed Pump Motor Driven Turbine Driven

Manufacturer : Weir Pump Ltd

Type : FK 4 E 36

No. of stages : 4+1 Kicker Stage

Direction of rotation viewed

on drive end

: Anticlockwise

SG at suction temperature : 0.901 0.901

Suction temperature oC : 164.6 164.6

Suction pressure bar : 20 19.83

Discharge pressure bar : 205.82 204.32

Differential pr. bar : 182.82 184.49

Differential Head m : 2103 2088

NPSHA above impeller eye m : 50.3 50.2

Flow rate m3/hr : 1080.3 1080.3

Leak-off flow m3/hr : 270 270

Efficiency % : 81.4 81.6

Speed rpm : 5705 5690

Power KW : 6830 6765

Turbine of BFP

Type : K 1401-2

Design output : 5589 KW

Max. output : 9123 KW

Normal Speed : 5330 rpm

Speed range : 2000-6030 rpm

Specified initial steam pressure : 7.182 ata

Exhaust pressure : 0.1 ata

Permissible deviation in initial Steam

pressure at No load

: 10.68 ata

Instantaneous deviation (12 Hrs/Annum) : 12.02 ata

Max. wheel chamber pressure permissible at : 10.15 ata

KORBA SIMULATOR

135

full load

Specified initial steam temperature : 300.6OC

Deviation without limitation : 322OC

Permissible deviation for longer period : 336OC

Permissible deviation for 400 Hrs.per annum

for not more than 15 minutes at a time.

: 336OC

Permissible deviation for 80 Hrs/annum for

not more than 15 minutes at a time

: 350OC

Specified cooling water temperature : 33OC

Start up time : 38 minutes

No. of stages : 14 Nos.

Type : Reaction

Axial thrust balance : By balance piston at steam

admission side and thrust

bearing.

No. of control valves : 5 Nos.

No. of stop valve : 1 No.

Aux. Control valve : 1 No.

Hydraulic speed senser : Primary oil pump

Electric speed sensor : Hall probes

Rotor Support : 2 Nos. Journal Bearing (Front &

Rear)

1 No. thrust bearing. (Front

Pedestal)

Over speed trip speed : 6330 rpm

Ist critical speed : 7550 rpm

Direction of rotation of steam flow : Anticlockwise (from direcion of

steam flow)

Turbine Auxiliaries

Main oil tank capacity : 6.3m3

Location : 0 meter level

KORBA SIMULATOR

136

Auxiliary oil pump : 2 Nos.

Discharge flow : 60 m3/hr

Rated head : 9 ata

Drive : Motor

Pump : +ve drive pump

D.C. Oil Pump

Nos. : 1 No.

Discharge flow : 16.25 m3/hr

Rated head : 2.5 ata

Type : Centrifugal pump

Jacking Oil Pumps :

Nos. : 1

Discharge flow : 0.54 m3/hr

Head : 100 ata

Drive : Motor

Pump : +ve drive pump

Voith Variable Speed Geared Coupling for Motor Driven BFP

BFP motor Speed : 1419 rpm

Gear ratio – I : 128/37

Gear ratio – II : 63/51

Primary speed : 4908 rpm

Full load slip : 2.6%

Max. output speed of the variable speed

geared turbo coupling.

: 5906 rpm

Regulating range : 4:1 downwards

Oil tank filling : 2500 litres

Filling pump Centrifugal pump)and Lub.

pump (Gear pump)

: Together driven as gear tooth

system drive via the pump shaft.

Aux. lub. pump : Three phase motor D 180 M

600/415V 22 KW, 50 Hz 3000

rpm

Lub. oil flow : 388 litres/min.

Boiler Feed Pump Drive Motor

KORBA SIMULATOR

137

Specifications

Type : Asynchroonous motor with

Squirrel cage rotor

Rating : 9800 KW

Speed : 1493 rpm

Stator voltage : 6.6 KV

Stator current : 987 amps

Frequency : 50 Hz

Limited Axial clearance (Max.) : + 2 mm

Lubrication

Bearing Lubrication Type : Oil ring and oil circulation

Oil requirement for both : 62 litres

TDBFP TECHNICAL DETAILS Turbine driven BFP uses a turbine of 14 stage connected to condenser Turbine is coupled with main pump having an engage/disengage unit called Power pack unit using oil pressure for above function. Between turbine and Booster pump gear assembly is there. In 500 MW unit there are two similar TDBFPS located on turbine floor. TDBFPS have a big LCP (Local control panel) having facility for all operations of TDBFP. Various system of TDBFP are discussed below : LUBE OIL SYSTEM Lub oil system of both TDBFPS are provided with one Main Oil Tank each in which oil level is separately maintained. It has two AC AOPs, one JOP AC and one DC AOP connected to tank. Lub oil pressure is maintained at 3.0 Kg/cm2 on throttling after the pump oil at discharge pressure which is 9.0 Kg/cm2 is called control oil which is used as governing oil. Lub oil after passing through coolers is led to various bearing of TDBFP system. DC AOP discharge oil is used on failure of AC AOPs bypass the coolers. During barring of TDBFP same lub oil at pr. 4.0 Kg/cm2 is used as power fluid in barring gear impellers. In the lub oil system provision is there to adjust the lub oil pr. by changing the recirculation flow.

KORBA SIMULATOR

138

SEAL INJECTION SYSTEM Mechanical seals are provided on BP side for which continuous cooling is done by CEP water. For BFP side constant seal injection pressure around 18 Kg/cm2 is maintained with the help of control valve. Filters are also provided in this line. From this line small pipe provides water in the exhaust steam as exhaust hood spray. Seals drain is collected as clean drain into drain tank and dirty drain flows into common drain header. In case of seal injection pr. low DC seal quench pump takes start and provides seal water from CST for safe coasting down of pump. Drain tank level is maintained separately with gland drain pumps. STEAM SYSTEM For TDBFP there are three sources for steam namely 1.Auxiliary steam 2.CRH 3.IP-LP cross-over steam. Extraction steam parameters are maintained at 4.0 Kg/cm2 & 300OC. During cold start when CRH or IP/LP steam is not available, Aux. steam is used for rolling of TDBFP for initial boiler filling. Once steam is insufficient for increasing the speed beyond 3500 rpm, CRH steam is used for further speed pick up. After IP - LP cross over steam is sufficiently available then Aux. steam, CRH steam is a automatically cut off, after closure of ACV. FW SYSTEM Water from Dearator is taken to the Booster pump. The discharge of the Booster pump is fed to BFP suction. From the discharge of the BFP the feed water passes through two numbers of high-pressure heaters, economizer and finally reaches the boiler drum. Drains & vents are provided in FW system for initial charging and venting of BFP during rolling. GLAND SEALING OF TURBINE Downstream steam from main turbine gland sealing is used for TDBFP sealing. Before opening of Exhaust valve of TDBFP gland sealing should be done as this line is connected to condenser. GOVERNING SYSTEM For having the required flow through BFP, speed of Turbine has to be adjusted and for that we need perfectly efficient governing system. Governing system is using the control oil at 9.0 Kg/cm2 which in turn depending on the position of starting device and speeder gear, will develop Secondary oil and Aux. secondary oil pressure for operating the 4MCVs and one ACV thereby adjusting the steam flow speed change is effected. In the Governing system separate governing filter is used. Governing is achieved with the help of two governors namely Hydraulic and Electrohydraulic

KORBA SIMULATOR

139

governors having EHG control and HG follows it up. On isolation of fluid line to EHG, Electro hydraulic Governor can be taken out of service. TDBFP speed control can be put on auto if EHG is selected. In this condition speed set point is automatically generated and feed water flow is maintained as per the requirement from Drum level controller. TDBFP speed Governing can be done either from UCB or from LCP or manually operating the Starting device or Speeder gear. TDBFP OPERATION Steps:

1. FW line charging

2. Recirculation valve lining up

3. Seal injection, exhaust hood spray charging

4. Gland sealing

5. Charging of lub oil system, starting of AOPs & JOP.

6. Barring valve Gearing. Barring speed is @ 200 rpm

7. Line up extraction steam lines

8. All drains provided in UCB and local to be opened.

9. First TDBFP resetting is done by making starting device 0% and ensuring that no. turbine trip condition should persist.

10. Speeder gear is made 100%

11. Hydraulic Governor is selected from console of starting device

positions,

12. Starting device position is increased slowly at 42% stop valve opens & indicated on the console.

13. At 56% MCV starts opening

14. Speed refrence should be kept manual. Increase speed reference

to 1000 rpm as soon as TDBFP picks up speed.

15. Ensure that EHC output is less than HG. Hence EHC is active

16. EHC is selected through key (key in released condition)

17. Pump venting is done at 1000, 2000, 3000 rpm

18. Ensure that the TDBFP is running normal.

KORBA SIMULATOR

140

19. Further speed reference is increased to pressure requirement, &

flow is monitored.

20. By adjusting the feed water master output TDBFP speed can be put on auto with the help of key. Subsequently, Feed water master can also kept on Auto.

OIL SUPPLY Oil is supplied to the tripping device through a filter and a solenoid valve. Low vacuum protection, high pressure protection, low lube oil protection and over speed tester are incorporated between the solenoid valve and the tripping device. The solenoid valve allows the flow of oil through it in the normal condition as per the specific requirement. It can be kept in either normally open or normally closed condition. When the solenoid valve is actuated due to any fault condition this will obstruct the flow of oil to the system connecting the down stream at the same time to drain. In the event of low vacuum, low lube oil or high back / extraction pressure corresponding protection gets actuated and they prevent the flow of oil to the system at the same time connecting the downstream side to drain. Oil after passing through these protections will pass through the overspeed tester and reaches the tripping device. In the working condition tripping device allows the oil to flow through it. Pressure on the downstream side will keep the tripping device in operating condition. When the tripping device trips due to overspeed, high axial shift or by manual operation it blocks the oil flow to the system at the same device it is required to keep it lifted till pressure is build up on the down stream side. Reengaging of the tripping device is possible remotely by providing additional equipment. Then the oil is supplied to the starting device, amplifier (s) nonreturn extraction valve (if present). STARTING Turbine is started by operating the starting device which opens the stop valve first without opening the control valves. By further operating the starting device after opening the stop valve, control valves open thus starting the turbine. After attaining the rated speed and the speed governor has taken over starting device is taken to the extreme position. SPEED CONTROL The speed governor compares the set point of the speed with the actual value and give an output in the form of displacement of the lever which holds the sleeve of the amplifier. This movement will result in a change of secondary oil pressure. This change will result in the change of servomotor position and consequently governing valve opening. This will lead to change of speed and a new balance is reached between the set point of the speed governor and the actual speed.

KORBA SIMULATOR

141

GOVERNING SYSTEM OPERATION OPERATION Adjust reference value of speed governor to minimum turbine speed, moves pilot valve of starting device downwards until it comes to a stop, i.e., in this position the rectangular block “a” in the functional diagram is shifted such that it is situated across the oil ducts shown in the diagram. During this operation the sliding guide bush on the amplifier is displaced, thus preventing secondary oil pressure being build up. After the hydraulic protection devices provided in the control circuit have been switched into their respective free passage positions, the rectangular block “a” of the emergency trip gear will be shifted into the position opposite the oil connections. When the TDBFP is being put into operation, trip oil is allowed to flow through the starting device into the space behind the piston of the emergency stop valve where it is fulfils the function of pressure oil for starting up. The piston will be tightly pressed upon the piston disc without allowing any leakage of oil. If the handwheel is now being turned in counter clockwise direction, the pilot valve of the starting device will be moved slowly upwards as represented by position “C” of the functional diagram. In this way, the trip oil will be directed also to the space in front of the piston disc while the space behind the piston is connected to the oil drain. As soon as sufficient pressure has build up in front of the piston disc, the starting device will be moved further in the upward direction, so that the pressure behind piston is going to drop. The resulting pressure differential will push the piston disc together with the piston upon which it is being pressed onto the ultimate position to the right. INCREASING THE TURBINE SPEED If the pilot valve of the starting device is brought into position “d” the sliding guide bush of the amplifier will be lowered further. This brings about an increase in secondary oil pressure with the result that the steam control valves are opened correspondingly, and the turbine rotor is picking up speed. After having attained its set minimum speed, speed control of the turbine will be taken over by the speed governor. Before the turbine speed is increased further by means of the speed governor, raise the pilot valve of the starting device right to the ultimate position at the top and block it there. DEVICE FOR REMOTE OPERATION OF EMERGENCY TRIP GEAR FROM THE CONTROL ROOM PURPOSE Starting a turbine from the control room requires in addition to the distant operation of the starting device, remote control of the emergency trip gear and as the case may be, also of an auxiliary slide valve. Such a control can be achieved with the help of the

KORBA SIMULATOR

142

following apparatus. Solenoid valve pressure switch , and an automatically opening control switch whose actuation will be blocked in the open position by an interlocking mechanism. SPEED GOVERNING SYSTEM PURPOSE The pressure oil governor Type SR IV controls the turbine speed and maintains it at a constant value in accordance with the functional relationship represented by its characteristic line. The control characteristic of the governor is of the proportional plus derivative type. The pressure of the secondary oil discharged at the governor output side forms the input signal that acts on the control piston of the servo valve. Mode of operation The signal transmitter for the speed governor is situated on the turbine rotor. The pressure of the primary oil thus generated by this transmitter will be a square function of turbine speed and forms the input signal (actual speed value) for the transducer. Any deviation of actual speed from a given reference value will therefore cause a deflection of the comparator lever which is functionally connected to the traducer. The force which the transducer exerts on the comparator lever is counteracted by a reference value spring. Under steady state conditions the spring force will equal the transducer force. The comparator lever is with its one end acting on the amplifier by means of a slidable bush. Both the amplifier and the bush are provided with discharge pots which, depending on the degree of their overlap, control the discharge of larger or lesser quantities of oil. When, e.g. owing to a decrease of primary oil pressure (Change in actual speed) the comparator lever is going down, the slidable bush will be displaced. This results in a smaller amount of overlap whereby the flow section for the oil discharge is reduced. The balance previously determining the volume of discharged oil will thus be disturbed. This causes an increase in secondary oil pressure so that the amplifier is going to follow the displacement of the slidable bush until the flow section of the discharge port will once more conform to the oil volume corresponding to the altered pressure ratio at the throttle. The pressure change depends on the stroke of the amplifier and on the characteristic of the tension spring. The functional relationship between the stroke of the amplifier and secondary oil pressure is roughly proportional. Secondary oil pressure is acting via a damping device as input signal for the control piston of the servo valve (actuator). Momentarily, the control piston will assume a position which is determined by secondary oil pressure, thus providing a path for the pressure oil to flow either to the upper side or to the underside of the servo piston in the actuator. As the servo piston is going downwards or upwards, the turbine control valves will be actuated accordingly. OPENING OR CLOSING OF TURBINE CONTROL VALVES When, owing to a decrease in turbine speed (actual speed change), the balance at the comparator lever is disturbed, secondary oil pressure will increase. The control piston in the servo valve (actuator) is going upwards under the influence of the increased

KORBA SIMULATOR

143

secondary oil pressure until a new balance between secondary oil pressure and the force of spring has been established. Depending on the value of secondary oil pressure and thus also on the position of the control piston in the servo valve (actuator) , pressure oil, although some what restricted by the effect of the throttle, will be admitted to the space above the actuator piston while the space underneath it is connected to the oil drain line (see the functional diagram : the rectangular block “c” will be displaced so that it is situated across the two oil connections). In this way, the piston is pressed downwards, thereby opening the turbine control valves. A feedback element on the actuator will restore the control piston the initial position represented in the diagram, thus stabilising the position of the actuator piston. If the turbine control valves have to be closed, i.e. when primary oil pressure is going up, the described sequence of steps will be followed in an analogous way, but with the opposite effect. SPEED SETTING The speed of the turbine can be adjusted within the limits of approx. 65 % and 107 % of rated speed for compressor drives and of 85 % to 107 % for generator drive sets. Depending on the model of speed setter provided, the adjustment can be effected either by hand, or by means of a pneumatically operated actuator, or by an electric motor. Changing the force exerted by the reference value spring (change in consequence that both the speed and the output is altered when the turboset is operated as an isolated unit, or the output alone with interconnected operation. The slope of the characteristic, and hence the field of proportional response, can be changed through appropriately, adjusting the feedback element in the valve actuator. GOVERNING SYSTEM - SPEED GOVERNOR PURPOSE The oil pressure set up by the governor impeller (primary oil pressure) serves for the control of the turbine speed via an hydraulic governor. The governor impeller is supplied with oil by a pump drawing from an oil tank. The additional pressure imparted to the oil by the impeller is a function for the turbine speed. The oil whose pressure has been thus boosted is supplied to the underside of pressure capsule in the hydraulic governor. MODE OF OPERATION The oil for bearing lubrication and for the impeller is supplied by the main oil pump via a screw valve and the appropriate channels and grooves in the housing the insert, and the bearing flange. Raidally drilled holes in the impeller are setting up a speed dependent pressure in the oil. A steady and adequate supply of oil to the radial holes in the impeller is ensured through the excess oil being collected by a collar and redirected to the housing. The pressure compartment surrounding the impeller is tightly sealed by the oil sealing rings which are held in position by retaining rings. The cover carrying the impeller housing is flangemounted to the front end of the front

KORBA SIMULATOR

144

bearing pedstal. The spur wheel and a spray nozzle for lubricating the gears are projecting into the bearing pedstal compartment. SPEED TELEMETERING A speed - calibrated pressure gauge (pressure gauge type tachometer) is provided for hydraulically measuring the turbine speed on the machine. It will be screwed into the tapped hole. Only slight modifications in the design of the shaft stub at the front end of the impeller and of the cover make it possible to mount devices for remote indication of the turbine speed. HYDRAULIC REFERENCE VALVE RELAY FUNCTION The turbine which drives the feed water pump is fed with steam from the main turbine. If the main turbine suffers an outage (e.g. emergency trip) the feed pump turbine takes its steam from the boiler reheater. The changeover to reheater operation must be performed without a major drop in speed in order to maintain the supply of feedwater. Since the feedwater regulator cannot perform the required alteration of the speed reference value as rapidly as needed, a step increase in the speed reference value is made. The reheater valve therefore receives a lift signal for the same steam flow that the turbine was receiving before the disturbance. After changeover the reference value relay is reduced slowly until the feedwater regulator can hold the lift signal for the reheater valve with no major disturbance (through adjustment of the speed reference). CONSTRUCTION AND MODE OF OPERATION The body of the reference value relay is incorporated in the governor housing and forms the link between the electrical reference setter and reference spring. The reference value relay is linked hydraulically to a 3/2 way solenoid valve (see also Governing System Diagram). When needed the solenoid valve is opened so that pressure oil can flow to the reference value relay. The pressure oil enters the body and passes through the control piston, which is bored and has elongated side ports, to the top of the step change piston. The step change piston is forced downwards as far as the adjusting screw permits and becomes one with it. At the same time the reference spring of the speed governor is tensioned through a rod connected to the step change piston. The turbine control valves move to maximum lift and the steam throughput is now regulated by the reheater valve.

KORBA SIMULATOR

145

After a pre-set delay the solenoid valve is deenergized and the oil spaces of the reference value relay are opened to drain. By means of a throttle the pressure is reduced slowly so that the feedwater regulator can perform the travel set by the reference value rely (step change) without difficulty. OPERATING CONDITION The initiating command for opening of the solenoid valve is a signal from the generator circuit breaker (tripped). The signal is interlocked if the check valve in the main turbine steam line to the feed pump turbine is closed and the reheater valve is already in an appropriate position (this is possible on overload during normal operation). AMPLIFIER PURPOSE The amplifier serves for converting the output signals from the speed governor and the extraction pressure controller into distinct values of secondary oil pressure which are passed on as input signal to the pilot valves of the respective control valve actuators. STARTING DEVICE SPEEDER GEAR AND SPEED CHANGER It is operated either manually or remotely by means of an electric motor. The rotary movement of shaft is transmitted via worm, to helical wheel. This helical wheel is arranged on the threaded portion of handwheel spindle and is fixed axially by inset and gear case. Spindle is connected with sleeve which is free to rotate in cover by feather key in such a force thrust rings which are guided in insert so tightly against sleeve, that sleeve and therefore spindle cannot turn and the spindle is moved in the axial direction when the helical wheel starts to rotate. Depending on the direction of rotation of helical wheel handwheel spindle can be moved in both directions until it bears against feather key and stop nut, respectively. In the two stop positions thrust rings act as a slipping clutch. In this case handwheel spindle and helical wheel rotate simultaneously without the handwheel spindle being moved in the axial direction. Even in the event of a fault, i.e. which the frictional resistance becomes too high, the device acts as slipping clutch and protects the electric motor against overload. PISTON The piston slides in a separate cylindrical housing of the actuator and is mounted on the piston rod. A conical follow up cam has been shrunk up on this piston rod and is secured by a pin. the upper end of the piston rod is screwed into the turnbuckle which also serves for attaching the tie rod which transmits the stroke of the piston to

KORBA SIMULATOR

146

the control valve leverage. The spaces above and below the piston are connected through internal ducts in the housing with the il passages of the pilot valve. PILOT VALVE The piston of the pilot valve is slideably mounted in a sleeve which is tightly inserted into the actuator housing. The core of the valve piston is drilled out and has access to annular grooves on the periphery which coincide with corresponding annular recesses in the valve sleeve. At the bottom the valve piston core is sealed by a screw plug, while at the upper end the oil is able to escape through radial passages which have been drilled into the wheel disc in a spider like configuration. In the region of the pressure oil inlet connection, four radial holes have been drilled from the periphery right down to the core of the valve piston. A pin on the top of the valve piston serves for locating a deep groove ball bearing intended for taking up the thrust. Above this thrust bearing a compression spring is provided which can be tensioned in conformity with the position of the bell crank lever by means of an adjusting screw. FEEDBACK SYSTEM The feedback system serves the purpose of stabilising the control movement. The piston rod is positively connected with the pilot valve via a bell crank lever. The cam follower mounted on the one arm of the bell crank lever is pressed against the conical follow up cam. The other end of the bell crank lever is connected to the adjustment screw of the pilot valve and thus is under the influence of the compression spring. MODE OF OPERATION Any change in pressure of the secondary oil results in an axial displacement of the valve piston. In this way, the relative position of the grooves on the valve piston surface with respect to the corresponding annular recesses in the valve sleeve will be altered with the result that pressure oil is admitted to the space below or above the actuator piston. At the same time, the return oil from these spaces is allowed to drain. The actuator piston will consequently make an appropriate movement and will operate the control valves by their leverage. Owing to the follow up cam, any movement of the actuator piston will be transmitted to the bell crank lever. the action of the bell crank lever on the compression spring goes in a direction opposite to the movement of the pilot valve. Under the influence of the spring force, the valve piston is thus going to assume an intermediate operating position.

KORBA SIMULATOR

147

ACTUATOR ADJUSTMENTS The functional relationship between secondary oil pressure and piston stroke can be changed by rotating the conical follow up cam about its axis. Such a displacement will after the interdependence between actuator piston stroke and compression spring preload while maintaining the direct proportionality between these two quantities. The preload on compression spring can be adjusted by means of the adjusting screw prior to such adjustments the cap has to be removed. PURPOSE The actuator serves for transmitting the positioning impulses for the control valves to the valve operating leverage. The lever system lifts or lowers the control valves of the turbine in such a way that the steam flow will always be adequate to the present or required turbine output. The pilot valve of the actuator receives its control impulses from the secondary oil circuit. However, the actual servo power for positioning the control valves is derived from the pressure of the oil which flows either to the top or to the underside of the actuator piston. MODE OF OPERATION Any change in secondary oil pressure brings about a corresponding stroke of the pilot piston. The annular grooves and oil pockets in the pilot and sleeves, respectively, are arranged in such a manner, that with increasing secondary oil pressure the pilot piston is moved upwards thus opening the pressure oil arriving at connection a channel for flowing to the upper side of the servo piston. By its resulting downward stroke, the piston is thus opening the control valves through the lever system. By means of a reset bar, the piston stroke is fed back to the lever via a bellcrank follower. The action of that lever on the compression spring goes in a direction opposite to the pilot piston stroke. Hence, the pilot piston is going to yield to the spring force and returns to its neutral position. The functional relationship between secondary oil pressure and piston choke can be changed by adjusting the inclination of the reset bar to the desired position with the help of a set screw. Such adjustments will affect only the amount of proportional gain while, with the design of reset bar here under consideration, the secondary oil pressure vs. piston stroke relationship will always a linear function. However, in cases where control requirements warrant it, it will be possible to provide also for non linear control characteristics through an appropriately shaped cam profile of the reset bar. The resetting mechanism serves for stabilising the control action. The functional chain extends between the piston rod and the pilot piston; these parts are interconnected by the bell crank follower and the lever. The pressure roller carried by one arm of the bell crank follower is pressed the reset bar. The other arm of the follower is connected with the lever and, through the adjusting screw, it transmits its motion to the compression spring.

KORBA SIMULATOR

148

START UP PROCEDURE FOR BFP DRIVE TURBINE 1. Ensure :

A. Oil supply to control oil system.

B. The manual valves to EHC and output of follow up piston are open.

C. Speed set point zero for energising Plunger coil.

D. No faults on EHTC.

E. Turbine trip reset.

2. Select the hydraulic governor with EHTC/Hydraulic selection switch.

3. Open the stop valve with starting device.The stroke of starting device is 0 - 53 %

for opening stop valves and bring main control valve secondary oil pressure to 1.5 kg/cm2.

4. Increase the stroke of starting device and observe that speed of the turbine

should not increase.

5. Raise the starting device position to 53 %.

6. Select the EHTC with selection switch EHTC/Hydraulic.

7. Increase the speed set point such that EHC position is more than starting device position.

8. Slowly bring down the EHC < HG in small steps, observe that the electronic

governor is active and starting device position is following the EHC position.

9. Increase the speed of the machine as per the start up curve manually. Raise/Lower speed reference push buttons till it is just below the BFP C set point.

10. Increase the speeder gear position to maximum. 11. Select the speed reference by BFP C with speed reference manual BFP C set

point ref. Selection switch.

12. Increase the speed of the machine till the BFP C set point becomes active i.e. manual set point is more than BFP C set point.

13. Observe the manual set point tracking the BFP C controller set point

14. To take the control from BFP C set point to manual set point.

KORBA SIMULATOR

149

A. Select the manual set point. B. Decrease in small step the manual reference such that manual set point

becomes active.

15. Change over to hydraulic governor.

A. Select the hydraulic governor.

B. Decrease the starting device position in small steps.

C. When HG < EHC observe the hydraulic governor active lamp glows.

D. Increase the speed set point to maximum to enable to operate the

hydraulic governor 100%.

INTERLOCK AND PROTECTIONS OF TDBFP TURBINE

1. Emergency trip from Control room L C P Local i.e. from Governing rack and turbine front pedestal all above trips can be resetted from UCB & LCP

Please note that mechanical trip can be resetted mechanically only by pressing emergency speed Governor testing device lever downward.

2. Exhaust steam temperature high trip Trip 120OC Alarm 110OC

3. Inlet steam pressure high alarm 7.0 kg/cm2, Trip 10.0 kg/cm2.

4. Governing oil pressure very low ,Alarm 6.5 kg/cm2, stand by pump start

At 4.5 kg/cm2. 5. Turbine vibration very high (displacement). Alarm 75µ, Trip 75µ

6. Axial shift high Alarm +/- 0.5 mm, Trip +/- 0.7 mm. (+) Exhaust side,

(-) Steam inlet side 7. Eccentricity high Alarm 75µ PP, Trip 75µ P.P.

8. Turbine differential Expansion high alarm +/- 2.5 mm

KORBA SIMULATOR

150

9. Lub oil pressure low. Alarm1.6 kg/cm2, trip1.0 kg/cm2. (Barring gear stops)

BFP PLEASE NOTE THE FOLLOWINGS All TDBFP protections will trip the turbine. Turbine can be resetted after getting all permissives for BFP and No turbine trip. But once turbine has been resetted it can be rolled even if any one of BFP permissives are not there. For example if turbine has been resetted and after that some one closes recirculation valve, turbine can be rolled. BFP suction valve limit switch are not provided therefore it has been bypassed. Seal quench water pressure low tripping has been provided from main header pressure. Therefore seal quench water pressure trip or permissive will not come into the picture if seal quench water to the Boiler feed pumps are isolated from their controller end. Emergency seal quench water pump start/stop knob (at MDBFP LCP) should be turn towards ‘OFF’ side after every tripping of Boiler feed pumps on seal quench water pressure low. Otherwise pump (ESP) will not take start on auto, in event of similar next tripping. DISENGAGABLE COUPLING -

• Engage - Zero speed & lub oil Pressure adequate.

• Disengage - less than 100 rpm. In both the cases power pack motor has to be stopped after completion of engage / disengage operation. However pump will take start on auto when engage or disengage command is given and conditions are fulfilled. In the same reference it is advisable to engage and disengage the coupling at zero speed only. BARRING GEAR

1. Barring gear valve opens on auto when speed is < 100 rpm. JOP will take start when barring gear valve.

2. Barring gear valve close on auto when turbine speed reaches 300 rpm. JOP

has to be stopped manually.

KORBA SIMULATOR

151

BFP T

UR

BIN

E G

LA

ND

ST

EA

M F

LO

W D

IAG

RA

M

TDBFP OPERATION

1. Open drain valves, DW 2A, DW 3A, DW 6A (from LCP), MSD - 81, MSD-32, MSD-83, MSD-66, MSD-67 - from LCP or control room.

(In case of TDBFP 6B open MSD 82, MSD 46, MSD-77, MSD-66, MSD 68)

2. Charge gland seal of Turbine and open Ex-21 (Exhaust to condenser)

KORBA SIMULATOR

152

3. Charge Aux. Steam and CRH steam to Turbine one after other by opening ASL 004 & Ex-20 (Ex 19 for TDBFP 6B) Atmospheric drain valves may be open to observe the draining of condensate in the Aux and CRH lines.

4. Open MV 78 and Ex 50 for heating the steam line upto ESV (Manual isolating

valve before ESV should be full open).

5. Watch steam inlet parameters and then open ESV for chest heating.

6. Roll the turbine according to start up procedures.

7. Closing drains & vents -

I. MV - 78 EX 50 before rolling.

II. Line drains - looking into steam parameters.

III. Turbine drains after attaining full speed and watching turbine parameters.

Booster pump and BFP are to be lined up according to the procedure followed in case of MDBFP.

SHUT DOWN OF TDBFP

1. Disengage the coupling at zero speed and put the machine on barring. Water in the booster pump is to be ensured while running the machine on barring.

2. Close Ex-21 TDBFP A

3. Isolate the gland steam to turbine seals when vacuum comes to 0.2 kg/cm2.

4. Isolate the steam lines.

5. Open the BFP vents and recirculation line vents to restrict suction/discharge

differential temperature.

6. Steam and water side isolations are to be done when shut down is for longer period.

7. Barring gear can be stopped after achieving casing temperature less than

100oC (It can be reensured by touching the turbine glands with hand).

KORBA SIMULATOR

153

BFP T

UR

BIN

E E

XT

RA

CT

ION

DE

TA

ILS

KORBA SIMULATOR

154

GOVERNING SYSTEM OF TURBINE DRIVEN BOILER FEED PUMP INTRODUCTION The main requirement of a boiler feed pump drive turbine (BFPT) governing system is to maintain speed depending on a signal from feed water controller which in turn depends on boiler feed pump delivery pressure and flow. The turbine speed is being controlled by an Electro hydraulic governor (EHG) which is constantly backed up by an another hydraulic governor. Speed control gangue of EHG is from 20 % to 110 % and that of Hydraulic governor (HG) is 50 % to 110 % of rated speed. The hydraulic governor comes into action immediately when the Electronic governor fails. For smooth change over an electronic tracking device is provided. The steam to BFPT is supplied from one of the bled steam line of main 500 MW steam turbine. During start up and shut-down of main turbine, steam is being supplied from cold reheat line. The BFPT governing system has a feature of starting the BFPT through steam from cold reheat line and then automatically switching on to bled steam supply as soon as it is available without any discontinuity. The other special features of BFP TURBINE governing system includes a quick closing stop valve, over speed trip, low lubrication oil protection, low vacuum protection, axial motion protection, remote tripping and remote engagement of emergency tripping device. HYDRAULIC GOVERNOR The hydraulic governor mainly consists of a governor impeller, hydraulic governor and hydraulic amplifier. The governor impeller is driven by BFPT shaft through gear wheels and it is supplied with a small quantity of oil from the main oil pump. Depending on the speed of BFPT, the governor impeller builds up a pressure on its periphery. This oil pressure called as primary oil pressure acts on a hydraulic governor bellow. The governor bellow is connected to a lever through a tappet. The force exerted on the bellow by primary oil pressure is transmitted to the lever through the tappet. A compression spring is mounted on the top of the lever which is pre-compressed by speeder gear. The signal from feed water controller through electronic governor determines the pre-compression of the speed set spring which balances the primary oil pressure acting on the bellow. The travel of the tappet is transmitted to the lever which is pivoted at one end through a joint. At its free end the pivoted lever is connected to the control sleeves. The control sleeves and follow-up pistons (amplifiers) are provided with ports. The overlap of these ports is dependent on primary oil pressure and speeder gear position. The follow-up pistons are held in a certain position by a spring. The pressure oil from trip oil circuit is admitted into the follow-up pistons through an orifice. The secondary

KORBA SIMULATOR

155

oil pressure for servomotors depends upon the gap between the follow-up pistons and control sleeves. Any variation in control signal from feed water controller will change the pre-compression of the speed set spring and thus disturbing the equilibrium of the tappet. This will change the port opening between follow-up pistons and control sleeves. The subsequent increase or decrease in the secondary oil pressures result in a displacement of servomotor pilot valves which determine the position of servopistons. The spindle of servopiston is connected to the control valves through a lever system. The feed back lever resets the pilot valves to the position corresponding to the required load even before the control action is completed. The follow-up pistons amplify the small pressure changes which are produced by governor impeller on a change in speed or due to feed water controller signal. It also reverses the direction of action of pressure in the secondary oil circuit. An interruption of this secondary oil leads to an immediate closure of control valves. The total control action of hydraulic governor can be summarised as follows : For examples, let signal from speed reference is increased, the control signal will try to compress the speed set spring and tappet will move down, which will result in increase in secondary oil pressure. The increased secondary oil pressure through servomotor opens the control valves more, allowing more steam to flow into the BFPT. As a result the speed of BFPT will increase till the primary oil pressure force and speed set spring force are again in balance. ELECTRONIC GOVERNOR The speed of BFPT is sensed by three hall probes. This speed signal through a pulse converter is fed to a speed governor which also receives another signal from a set point setter. Any change in speed of BFPT or set point, speed governor will give signal to an electronic amplifier. The output signal of electronic amplifier is given to the moving coil measuring system of a electro-hydraulic converter. EHC converts the electric signal into a hydraulic signal. The feed back action of EHC is effected by a differential transformer mounted on EHC power piston and feed back amplifier. The power piston EHC through levers move control sleeves of two hydraulic amplifiers. The construction and action of these hydraulic amplifiers are exactly same as that described under hydraulic governor. The output of these hydraulic amplifiers i.e. secondary oil pressure is connected in a parallel with the output of the hydraulic

KORBA SIMULATOR

156

governor amplifiers. Thus the secondary oil pressure leading to servomotors is controlled both by hydraulic governor and electronic governor (Function of a low signal selector). A fail safe is provided in the electronic governor to hold the last signal to electronic governor from BFPT controller. In case the signal from BFPT controller fails, and therefore the turbine speed will be maintained at that operating point. BFPT SPEED CONTROL BY ELECTRONIC AND HYDRAULIC GOVERNOR In the present governing system, hydraulic governor acts as secondary governor to electronic governor. As mentioned earlier that the output of both the governors controls the secondary oil pressure. But as a fact the governor which gives less secondary oil pressure will control the final secondary oil pressure. Since the hydraulic governor is designed to follow the electronic governor constantly, the hydraulic governor is set at slightly higher secondary oil pressure. Therefore always electronic governor will be controlling the secondary oil pressure and hydraulic governor will follow. The controlling range of electronic governor is from 20 % to 100 % and that of hydraulic governor is from 50 % to 110 % when electronic governor fails in the range of 50 % to 110 % the speed of the turbine will rise momentarily corresponding to hydraulic governor setting and then hydraulic governor will hold the speed and further speed can be controlled manually. In case the electronic governor fails in the range of 20 % to 50% of speed, the speed will be immediately set automatically to 20 % through the starting device. On the top of starting device a hydraulic reference valve relay is mounted, which receives the pressure oil through the solenoid valve. The solenoid valve gets energised only when BFPT set speed is less than 50 % and electronic governor fails. This signal is also given to the starting device motor, which slowly moves towards 20% speed setting. Through the solenoid valve and hydraulic reference valve relay the starting device is set immediately to 20 % speed, when starting device motor has moved to 20 % speed setting, the actuating signal to solenoid valve and motor are disconnected and turbine runs at 20 % speed as long as it is not disturbed. Further control of turbine can be achieved by operating the starting device manually. STARTING OF BFPT During normal running, the steam to BFPT is supplied from one of the bled steam line of main 500 MW turbine. During starting and shut-down of 500 MW turbine, the steam to BFPT is supplied from the cold reheat line.

KORBA SIMULATOR

157

TD

BFP G

OV

ER

NIN

G S

CH

EM

E

KORBA SIMULATOR

158

Steam into turbine enters through main stop valve and main control valves. The auxiliary control valve and servomotor are located in the cold reheat line before the main stop valve. The main stop valve is of quick closing type. The valve is actuated by means of a starting device. The stop valve consists of a spring loaded piston and piston disc, which is connected to the valve cone through a spindle. For opening the stop vlave, start up oil (from starting device) is admitted to the space above the spring loaded piston, by operating the starting device. Due to start-up oil pressure, the piston moves towards the piston disc and they from a tight seal against each other. Oil from trip oil circuit is then admitted to the space under the piston disc and the space above the piston is connected to oil drain. The trip oil now forces both piston disc and piston to the outer position thereby opening the stop valve. As long as the trip oil pressure is maintained the piston and the piston disc cannot be separated by spring force. The stop valve is closed only when the trip oil pressure drops substantially. On a loss of trip oil, the trip oil pressure, the pressure of secondary oil tapped from trip oil circuit, drops to zero, thus causing the main control valves and auxiliary control valve to close. This arrangement provided a two fold protection against steam entering the turbine. Provision is made for on load testing of stop valve. To admit the steam into the turbine, HP control valves must open since during start, is no bled steam available, steam is to be supplied from cold reheat line and for this Aux. Control valves is to be opened. The arrangement and design is such that aux. Control valve will open only when main control valves are fully opened. With the help of starting device the hydraulic governor is manipulated to increase first the main secondary oil pressure then aux. secondary oil pressure. During this time electronic governor is set at 20 % or speed. Once the aux. control valve is open, steam from cold reheat or Aux. Steam line is available to turbine and after picking up the speed, electronic and hydraulic governor will take over. When bled steam is made available, aux. control valve will close automatically if available steam from IP, LP cross over is sufficeint and turbine will be operating on bled steam supply. DISTURBING PROCESS SIGNAL UNIT (DPSU) When main turbine is tripps bled steam to drive BFPT will not be available and BFPT has to derive the steam from cold reheat line. As soon as the bled steam supply is stopped, the turbine (BFPT) governing system will open the control valves more and more. Thus main control valves immediately open fully and then aux. control valves starts opening till the required steam quantity is met through cold reheat line. In process, when main control valves open, very fast response from electronic governor is expected and will interfere with the hydraulic governor which is comparatively slow. To avoid this interference, a disturbing process signal unit is used to set the hydraulic governor to the maximum valve opening position as soon as 500 MW turbine is tripped. 500 MW turbine trip signal is given to a solenoid valve cylinder of D.P.S.U. The downward movement of the piston compresses the speeder gear to maximum

KORBA SIMULATOR

159

speed setting and thus building maximum secondary oil pressure corresponding to full valve opening. FEED WATER HEATERS INTRODUCTION A feedwater heater is a special form of a shell and tube heat exchanger designed for the unique application of recovering the heat from the turbine extraction steam by preheating the boiler feedwater. Its principal parts are a channel and tubesheet, tubes, and a shell. The tubes may be either bent tubes or straight tubes. Feedwater heaters are defined as high pressure heaters when they are located in the feedwater circuit upstream from the high pressure feedwater pump. Low pressure feedwater heaters are located upstream from the condensate pump which takes its suction from the condenser hotwell. Because the discharge pressure from these pumps differs greatly, the physical and thermal characteristics of high and low pressure feedwater heaters are vastly different. Typically low pressure feedwater heaters are designed for feedwater pressures between 27 kg/cm sq. and 56 kg/cm 2, high pressure feedwater heaters range from 112 kg/cm2 for nuclear heat sources to 335 kg/cm2. for super critical boilers. Regardless of the actual design pressure, the classification depends upon the cycle location relative to the feedwater pumps. The design pressure is specified sufficiently high so as to not over-pressure the channel side of the heaters under any of the various operating conditions, particularly cat pump shut-off. Each feedwater heater bundle will contain from one to three separate heat transfer areas or zones. These are condensing, desuperheating and sub-cooling zones. Economics of design will determine what combination of the three is provided in each heater. A condensing zone is present in all feedwater heaters. Large volumes of steam are condensed in this zone and most of the heat is transferred here. The desuperheating zone is a separate heat exchanger contained within the heater shell. This zone’s purpose is to remove superheat present in the steam. Because of the high steam velocities employed, condensation within the desuperheating zone is undesirable. The sub-cooling zone, like the desuperheating zone, is another separate counter flow heat exchanger whose purpose is to sub cool incoming drains and steam condensate.

KORBA SIMULATOR

160

HEATER OPERATION The following are precautions that should be adopted when operating these feedwater heaters. START - UP Feedwater heater operation should not be undertaken if any of the protective devices are known to be faulty. Feedwater heaters are not to be operated at fluid temperature higher than those shown on the specification sheet. Feedwater heaters must not be subjected to abrupt temperature fluctuations. Hot fluid must not be introduced rapidly when the heater is cold, not cold fluid when the heater is hot. Prior to opening the feedwater valve, the channel start-up vents are to be opened and remain open until all passages have been purged and feedwater begins to discharge. To remove air from the shellsides of a heater which does not operate under vacuum, the shell start-up vent valves should be opened prior to the admission of steam to the feedwater heater. The extraction lines must be free of all condensate to prevent damage to the heater internals by slug flow. When the drains outlet valve is opened, the shell start-up vent valves are to be closed and the operating air vent valves are to be opened. Continuous venting of air and other non-condensibles is assured by keeping the shell operating vent valves open. On initial plant start-up of horizontal feedwater heaters, having integral drain coolers, the liquid level is to be kept just below the high level alarm point. This will avoid the possibility of flashing at the sub-cooler inlet and the possible tube damage that can result. During initial start-up phases, the drains approach temperature (difference between drain cooler and feedwater inlet temperatures) should be monitored. Approach temperatures in excess of 8OC indicate the probability of flashing at the sub-cooler inlet. In this case, the liquid level should be raised until the drains approach temperature approached the specified value. The various turbine extraction’s are charged as follows :

• LPH – 1, Always in service.

• LPH - 2 & 3, These extractions are charged when the unit load is around 50 MW.

• Deaerator Extraction is charged when IP exhaust pressure is > 3.5 kg/cm2.

(around 40 % of unit capacity)

• HPH - 5 & 6 These extractions are charged when unit load is around 50 to 60 % of unit capacity.

KORBA SIMULATOR

161

VENTING Proper venting is necessary on feedwater heaters. All operating air vent connections must be piped to permit continuos venting. The venting system in a feedwater heater is designed to assure that all points where non-condensable gases could collect are vented. Failure to utilise all of the operating air vents can lead to corrosion damage and/or loss of performance due to air blanketing. Vent lines of heaters operating at a different shell pressure must not be piped to a common manifold. Failure to run individual vent lines from each heater has resulted in inadequate or no venting of a heater operating at a lower shell side pressure than other heaters, piped into a common manifold. Also, tubing adjacent to an air removal connection has failed due to erosion caused by blow back of condensed vapour fed into the heater from a manifold which also served a heater operating at a higher shellside pressure. Multiple operating air vent connections on the same heater can be manifolded downstream from the vent orifices and exhausted through a single vent pipe. The manifold must be sized to handle the total flow from all vents and discharged into a pressure lower than the vent pressure. A sharp distinction must be drawn between start-up vents and operating air vents, as identified on the Setting Plan drawings. Start up vents must be closed during operation, and in no event are start up vents to be piped up to a manifold serving the operating air vents. Vents should not be cascaded. Vent flow control is best accomplished through the use of properly sized sharp edged orifice constructed of stainless steel or other suitable erosion/corrosion resistant material. Vent piping should be sized to assure that the back pressure, at the discharge of the vent orifices, is no greater than 50 % of the shellside operating pressure. When there is no desuperheating zone in a given heater, this shellside operating pressure may be considered as equal to the steam inlet pressure. If a desuperheating zone is employed, deduct 0. 35 kg/cm2 from the steam inlet pressure to obtain the approximate shellside operating pressure for the purpose of sizing vent piping. Note that the intent is to control the operating air vent flow by assuring critical flow through the vent orifices. Isolation valves in vent piping should either be locked in the open position or some other suitable means provided to assure that such valves cannot be closed during normal operation.

KORBA SIMULATOR

162

FEEDWATER BY - PASSING A feedwater heater may be severely damaged by erosion and/or vibration, if it is operated for any significant period or time with the next lower heater’s feedwater flow by passed. When a heater is by-passed, its normal feedwater is passed on to the next higher heater. This next higher heater will come close to making up the duty of both heaters. This single heater will tend to draw a total amount of extraction of steam approximately equal to the flow to both heaters. In the case of heaters with desuperheating zones, the increased steam load due to by-passing the previous heater can cause an excessive pressure drop in the desuperheating zone, which in turn can cause condensation. The condensate flowing at high velocity can lead to severe tube erosion. Excessive steam flow to a heater, resulting from by-passing the feedwater side of the previous heater, can result in :

• Localised high velocity leading to vibration of the tubing.

• Flows which cannot be adequately handled by the drain control valve.

• Condensation in the desuperheat zone and high velocity impingement. DEAERATORS PRINCIPLE OF DEAERATION The deaerating heater utilises steam by spraying the incoming water into an atmosphere of steam in the preheater section (first stage). It then mixes this water with fresh incoming steam in the Deaerator section. (Second stage). In the first stage the water is heated to within 2O of steam saturation temperature and virtually all of the oxygen and free carbon dioxide are removed. This is accomplished by spraying the water through self adjusting spray valves which are designed to produce a uniform spray film under all conditions of load and consequently a constant temperature and uniform gas removal is obtained at this point. From the first stage the preheated water containing minute traces of dissolved gases flows into the second stage. This section consists of either a distributor or several assemblies of trays. Here the water is in intimate contact with an excess of fresh gas/free steam. The steam passes into this stage and it is mixed with the preheated water. Deaeration is accomplished at all rates of flow if conditions are maintained in accordance with design criteria. Very little steam is condensed here as the incoming

KORBA SIMULATOR

163

DE

AE

RA

TO

R A

ND

FST

CO

NN

EC

TIO

N

KORBA SIMULATOR

164

water has a high temperature caused by the preheating. The steam then rises to the first stage and carries small traces of residual gases. In the first stage most of the steam is condensed and the remaining gases pass to the vent where the non-condensable gases flow to the atmosphere. A very small amount of steam is also discharged to the atmosphere which assures that the deaerating water is adequately vented at all times. The water which leaves the second stage falls to the storage tank where it is stored for use. At this time the water is completely deaerated and is heated to the saturated steam temperature corresponding to the pressure within the vessel. The condensate pressure just before the entry to Deaerator shall be atleast 3 psi more than the Deaerator steam pressure. OPERATION OF THE DEAERATOR

1. Flush out all lines and tanks with water until there is no apparent indication of foreign matter or rust. Spray nozzles should be free from all foreign material.

2. Manually check all controls to see that each is working freely.

3. Check to see that all instruments are operating and indicating correctly.

4. Open the Deaerator vent valves or open orifice bypasses to atmosphere.

5. Admit condensate water and slowly increase from 15 % to 30 % of design inlet

flow fate.

6. Put one boiler feed pump in service with recirculation to Deaerator.

7. After making certain that adequate steam pressure is available, open steam valve slowly admitting steam into the Deaerator.

8. When a strong flow of steam issues from the vents, start throttling the vent and

check feed storage tank temperature.

9. The gauge should read 2 to 3oF below saturation temperature at the existing pressure.

10. Throttle back vent valves to operating positions. The final position is

determined in conjunction with oxygen tests during operation.

KORBA SIMULATOR

165

RECOMMENDED NORMS FOR TEMPERATURE CHANGES Changing cold or hot water admission to the water box must be accomplished in a controlled manner. The control must assure that the rate of temperature change of the metal in the shell or the water box does not exceed 400oF per hour with instantaneous changes not greater than 50OF per minute for a total excursion of 150oF. Cold start-up can severely stress a Deaerator. To avoid a severe thermal shock it is recommended that cold start ups be preceded by a warm up steam with the vents open and no flow in to the water box. The steam flow should be regulated to permit the steel shells to heat at a rate of 50OF per minute upto about 200oF. Water in the storage tank should also be heated to the same value. When the entire vessel and its contains are heated, the steam supply should be shut off and any remaining steam vapour should be vented. Accelerated cooling is often desirable for repair work. Accelerated cooling can be accomplished using a cooling fluid which is 100OF to 150OF lower than the metal temperature until the metal has cooled to about 250OF. The rate of change of metal temperature should stay in the 100O F/Hr range. Once the metal is at or below 250OF, cooling of 60OF to 70OF may be used. FEED SYSTEM OPERATION SYSTEM DESCRIPTION The purpose of the Feedwater System is to provide an adequate flow of properly heated and conditioned water to the boiler and maintain boiler drum level compatible with the boiler load. This system also conveys water to the boiler reheater at temperators, superheater at temperators, auxiliary steam desuperheaters, the high pressure bypass desuperheater and HP fill & purge SGCW pump cooler. Feedwater is heated to achieve an efficient thermodynamic cycle. Under normal operating conditions, feedwater flows from the outlet nozzles of the Deaerator storage tank, through the boiler feed booster pumps (BFBP), to the boiler feed pumps (BFP). From the discharge of the boiler feed pumps, the flow continues through both high pressure feedwater heater strings to the boiler economiser inlets. A bypass line around the heaters is provided for removal of either or both heater strings from service.

KORBA SIMULATOR

166

The high pressure feedwater heaters receive extraction steam from the cold reheat line and the IP turbine. The feedwater absorbs heat from the extraction steam as it passes through the heaters. The boiler feedwater flows through individual suction line to the Booster Pumps of 2 x 50 % turbine driven boiler feed pump ( A & B) and 1 x 50 % motor driven boiler feed pump (C). A hand operated isolation valve followed by a temporary start up strainer have been located in each booster suction line. At the suction line of booster pump C (Motor driven BFP) there is provision for introducing hydrazine and ammonia which could be dozed during a wet lay pump operation. From the discharge of booster pump, feedwater flows through the suction of BFP. A flow element has been located in each of these lines. One 275 mm inner dia discharge line merges out from each BFP with one check valve and one motor operated stop check valve placed in series. These lines then form a common discharge line. Each feed pump discharge is provided with an automatic modulating recirculation valve and two locked open isolating valves, located before the discharge check valve, and recirculate feedwater back to the Deaerator feedwater storage tank, as required. This ensures that the feed pumps are each protected by maintain the minimum recirculation flow corresponding to its speed. The balance leak-off line of each BFP is always open and ensure a return flow path to the pump suction. The feedwater temperature is initially raised by passing through the low pressure feedwater heaters and Deaerator feedwater heater. These heaters are associated with the condensate system. The feedwater temperature is further raised in high pressure feedwater heaters. The common discharge header of BFPs ultimately splits into three lines: one for each string of high pressure feedwater heaters (HP Htrs. 5A-6A & 5B-6B) and a common bypass line for both strings. The feedlines to HP feedwater heaters each contain two motor operated isolation valves: one on the inlet side of HP feedwater heater 5A/5B and one on the outlet side of HP feedwater heater 6A/6B. The bypass line located between heaters 5A-6A string and heaters 5B-6B string and is sized to accept full feedwater flow, which can be regulated by a motor operated globe valve. Pressure relief valves are provided on the tube side of heaters, to accommodate thermal expansion of the feedwater when a heater is isolated. The outlet header from each string the joins into a common header. The low-load feed control valve (FCV-0657) together with one motor-operated control valve bypass and two motor operated control valve inlet -outlet isolation are located parallel to the above common header. Thereafter, this header contains a flow element, a check valve and an isolation valve prior to entering into the economiser.The reheater attemperator supply line has been taken from an intermediate stage of each BFP which join into a common line prior to entering into the spray control station. This line also contains a flow element. The arrangement for superheater attemperator supply line is identical to the above one, excepting that the line is taken from the kicker stage of each BFP.

KORBA SIMULATOR

167

HP H

EA

TE

R C

ON

NE

CT

ION

S

KORBA SIMULATOR

168

SYSTEM CONTROL Two x 50 % turbine driven boiler feed pumps can be controlled either from the Unit Control Room (UCB) or from the Local Control Panel (LCP). These pumps are used during normal operation. In addition, another 50 % motor driven boiler feed pumps has been provided for start-up and standby operation, which is to be controlled from the Central Control Room. Under normal operating conditions, the main feedwater flow continues through each of the two turbine-driven BFBP discharges to the BFP associated with a particular BFBP. There is no cross connection between BFBP’s, nor is there any isolation valve between the discharge of the BFBP ‘s and the suction of the BFP’s. Under normal operation, if there is a malfunction with either a BFBP or BFP, that unit will trip out and cause its companion pump to also trip; at the same time, the motor - driven BFP will be brought up to system performance level by operator intervention. Suction strainer of each BFP is provided with a diff. Press. Switch (PDS-0601, 0612 & 0623). In the event of high differential pressure (0.5 kg/cm2 ) these switches actuate control room alarm. This alarm indicates that the suction strainer is dirty and should be cleaned as soon as possible. The standby pump should be cleaned as soon as possible. The standby pump should be placed in service. Thereafter, the suction strainer should be thoroughly cleaned. Each BFP is protected by safety interlocks to prevent eventualities like dry running, low NPSH, lubrication failure, minimum flow conditions, etc. Steam to BFP turbine is normally provided from the IP turbine exhaust. However, during unit start-up or during main turbine tripped condition, steam to BFP turbine is supplied from auxiliary steam or cold reheat steam. Each TD BFP has been provided with two x 100 % Main / Auxiliary Oil Pump and one Emergency Lube Oil Pump. These pumps can be controlled either from UCB or from LCP. The standby auxiliary oil pump shall start automatically in the event of either the tripping of the running oil pump or fall in pump discharge header pressure, to 6.5 kg/cm2, sensed by the pressure switch PSL 3.1. The emergency lube oil pump shall start automatically when lube oil pressure falls to 1.0 kg/cm2, sensed by pressure switches (PSL 3.2A, B & C). This pump can also be controlled either from UCB or from LCP. The jacking oil pump control has been provided on LCP. This pump shall start automatically when turning gear motor operated valve is open. Control of ID TD BFP turning gear has also been provided on LCP. Turning gear motor operated valve shall open automatically when BFP turbine speed is less then 210 rpm and shall close automatically when BFP turbine speed is greater than 240 rpm. The opening of the valve is so controlled that during turning gear operation the turbine speed remains around 100 rpm. Further, the above valve can only be opened either automatically or manually provided the lube oil pressure is 2.5 kg/cm2 (PSL 3.11) or more.

KORBA SIMULATOR

169

Each BFP turbine has been provided with Electro hydraulic governor (EHG), which is backed by a hydraulic governor. EHG can be controlled from both UCB and LCP. EHG is actuated to maintain turbine speed depending on a signal from feed water controller. Speed of MDBFP is maintained by actuating the scoop tube of the hydraulic coupling, depending on the feedwater controller signal. The MDBFP has been provided with one auxiliary oil pump and one shaft driven oil pump associated with the hydraulic coupling. MDBFP utilises the auxiliary lube oil system until the BFP’s associated shaft driven oil pump has developed an acceptable discharge pressure. The auxiliary lube oil pump is then placed in standby. The control of auxiliary oil pump has been provided on UCB only. This pump shall start automatically under any of the following conditions :

a. When start command is initiated to MD BFP.

b. When MD BFP rotates in reverse direction during standby condition.

c. In the event of tripping of MD BFP for a duration of 5 to 10 minutes.

d. When lube oil pressure falls to or below 1.2 atg, sensed by pressure switch and MDBFP is running.

This pump shall stop automatically when lube oil pressure increases to 3 kg/cm2, sensed by pressure switch. Seal water for both MD and BFP’s is normally supplied from CEP discharge header. Over and above this supply, a common Emergency DC Seal Water Pump (ESP) for all BFPs has been provided to supply water to BFP seals in the event of normal supply failure. The control of ESP is located on the LCP. When the quench pressure to BFP seals falls to 10 kg/cm2, sensed by pressure switches , all BFP shall trip and ESP shall start automatically. Once started, ESP shall be stopped after 5 minutes. Seal water pressure is maintained by actuating the diff. pressure control valve CI-7, 13 & 19 depending on the different pressure signal (sensed by DPT - 244a, b, c) between seal water supply header and Booster pump A/B/C suction line. Gland seal drains of all BFPs is collected in a gland drain tank, where from the drain is transferred by 2 x 100 % Gland Drain Pump (GDP) to Condenser via LP Flash Tank. The GDP is controlled from the LCP. The Gland Drain Pump shall start automatically when gland drain tank level becomes high and shall stop automatically when above level becomes low (both level sensed by LS 162.) Recirculation Control Valve of each BFP modulates automatically to ensure minimum recirculation flow (sensed by flow transmitters corresponding to the operating speed through the pump. This valve opens automatically when differential temperature of feedwater across BFP suction and discharge increase 10OC.

KORBA SIMULATOR

170

The boiler feed pumps each discharge through a motor operated discharge valve. During normal operation FW passes through the HP feedwater heaters. However, in the event of very high level in individual heater, one string of HP feedwater heaters, gets bypassed. For this reason motor operated bypass valve is placed across two strings of HP feedwater heaters. A common SERVICE - AUTO - BYPASS control switch for HP feedwater heater bypass arrangement has been provided on UCB. On AUTO position of the control switch, in the event of very high water level in any HP feedwater heater, sensed by level switches, the bypass valve opens around 50 %. Opening of FW-014 to 50 % position shall cause closing of inlet - outlet motor operated valves of the affected string. When both the string of HP feedwater heaters get bypassed, FW-014 opens full and maintains normal feedwater flow. Full opening of FW-014 shall cause closing of inlet-outlet motor operated valves of the string in service. Restoration of the heater level to normal will not automatically restore the affected string into service. The return to normal operation must be initiated by operator action with the help of SERVICE position of control switch. On BYPASS position of the control switch both heater strings get bypassed, as explained above, irrespective of level condition in each heater. After passing through the HP feedwater heaters, the feedwater reaches the feed control station. During start-up and low load (20 % MCR) condition, l Element drum level control remains in service and the low load feed control valve, isolating valves open automatically. Full opening of these valves shall cause closing of feed control station bypass valve, FW-015. When the load increases and exceeds 20 % MCR, 3 element drum level control comes into service and the bypass valve opens automatically. Full opening of this valves has been provided on UCB. The feedwater header is then directed to the economiser inlet header. INSTRUMENTATION AND CONTROLS FOR FEED WATER DESCRIPTION A 3 element Deaerator feedwater storage tank level control with condensate flow matched to the feedwater flow trimmed from the Deaerator level error system, is supplied. This system has provisions for an automatic trip of all feedwater system pumps in the event of every low level in the Deaerator, with low level alarms at a higher level. Dissolved Oxygen at BFBP suction and dissolved Oxygen, PH, Conductivity & residual hydrazine at economiser inlet are monitored. Analysers and recorders are provided in the steam and water analysis system panel. The BFBP’s are driven by the same drives as that of the BFP’s, and as such, operate simultaneously and in the same mode as their respective BFP’s. Differential pressure across the BFBP suction strainer is monitored. These strainers and their associated instrumentation are temporary and should be removed prior to

KORBA SIMULATOR

171

commencement of commercial operation. A high differential pressure across the strainer will initiate an alarm in the control room so that the pump in question can be taken out of service and the strainer cleaned. The speed of each BFP turbine is regulated by the 3-element feedwater control which is regulated according to boiler drum level, feedwater flow to the economiser and main steam flow. During low load (around 20 % MCR) conditions, the system is regulated only by drum level (single element control) by modulating the low load feed control valve. There are no control valves in the main discharge piping from the BFP to the boiler, since the Feedwater flow is controlled by the BFP turbine speed or in case of the motor driven pump by the hydraulic coupling scoop tube position. MOTOR DRIVEN PUMP If the motor driven pump is used for start-up, there is no back pressure available through the boiler and the hydraulic coupling has only a 4:1 turndown capability. Therefore, the low load feed control valve is provided to prevent an inability to control the pump during start-up. TURBINE - DRIVEN B.F. PUMP OPERATION Either of the turbine -driven pumps has the capability to be used for starting up the plant. An auxiliary steam supply to each pump has enabled either pump to run up to a point where sufficient flow is available to the boiler. The low-load feed control valve will regulate flow to maintain drum level. SINGLE ELEMENT VERSUS 3 - ELEMENT CONTROL A common nominal 20 % low load feed control valve has been installed in parallel to the common pump discharge header which will operate from a single element drum level signal. When the 20 % load index is reached, the control will transfer, and the low-load feed control valve is to be closed manually. The flow will then be directed through the main discharge line with the control transferred to the normal 3-element feedwater control signal which will modulate the hydraulic coupling scoop tube for the motor-driven pump or the turbine governor for the turbine driven pumps. The flow through each BFP is measured in the pump suction line. Control valves are provided in recirculation lines from each pump discharge to the Deaerator, to insure the minimum flow through each pump corresponding to its speed. Feedwater heaters 5A-6A & 5B -6B all have automatic bypassing on very high level incorporated in their controls.

KORBA SIMULATOR

172

The inlet and outlet motor operated block valves (MOV) on each string of heaters and the heater bypass MOV are interlocked to prevent complete shut off of feedwater flow to the boiler. Very high level in either heater in a particular string (5A/6A or 5B/6B) will initiate isolation of that string and opening of the bypass to 50 % positions. HEATER DRIP AND VENT OPERATION DESCRIPTION OF SYSTEM The purpose of the Heater Vents and Drains System is to remove condensate that has accumulated in the shell side of the closed feedwater heaters from their heat source the extraction steam, and cascade the condensate to the next lower pressure heater. This system also removes any non-condensable feedwater heaters. The heater drain system transfers the shell drains from each closed feedwater heater to the next lower stage of heating and ultimately to the condenser through HP/LP Flash Tank. The normal operating flow path is from heater nos. 6A-6B to No. 5A-5B and on to the Deaerator where in the drains are incorporated in the feedwater flow. From heater No. 3 the drains cascade to heater nos. 2,1, through the drain cooler and into the condenser, through LP flash tank. The drain line from the drain cooler section of each heater, except heater no. 1, divides into two branches, one leading to the next lower pressure heater and the other to HP/LP Flash Tanks. Each branch contains a modulating type control valve located near the inlet of the receiving vessel. The drain from heater no. 1 has a separate drain line to the LP flash tank, through a modulating control valve. Vents and drain of HP/LP flash tanks are finally connected to the condenser. Normal drain from each closed feedwater heater except LP heater 1 has also been provided with a local manually operated level control bypass valve, which can be used in the event of controller failure. All HP feedwater heaters 6A-6B & 5A-5B are provided with shell operating vents and shell start-up vents, which are routed to the HP flash tank manifold, where from drain and vent are connected to the condenser. Likewise, LP feedwater heaters 2 & 3 are provided with shell operating vents and shell start-up vents, which are directly routed to the condenser along with LP Flash Tank vent line. LP feedwater heater no.1, however, is provided with shell operating vent alone, which is also routed to the condenser. The operating vent lines from all feedwater heaters are fitted with orifice plates for flow control. The start-up vent lines from feedwater heater nos. 6A-6B, 5A-5B, LPH-3 & LPH-2 and deaerating feedwater heater are provided with manually operated modulating valves. The deaerating feedwater heater vents, both start-up and operating, are piped to atmosphere.

KORBA SIMULATOR

173

SYSTEM CONTROL All feedwater heaters are provided with two field mounted pneumatic level controllers, one for each drain flow path. One controller is set to maintain normal water level in the heater, by providing an appropriate control signal for the modulating drain control valve, located in the cascading drain line to the next lower pressure feedwater heater. The second controller is set at a level higher than normal, and provides a control signal to an alternate modulating drain control valve, which is located in the line branching off the cascading drain and connected directly to the HP/LP flash tanks. The normal operating level of each feedwater heater is specified by the heater manufacturer and this define the set point for the normal level control. Each feedwater heater is provided with “low”, “high” and “high high” level switches. The “low” level switch, set below the lowest pneumatic level controller set point, initiates an alarm in the control room so as to indicate the normal level controller malfunction. The “high” level switch, set above the highest pneumatic level controller set point, provides control room annunciation of high condensate level in the heater. Also, the “high” level switch will open the modulating control valve in the emergency drain line of the affected heater. The “high high” level switch which has a set-point above the “high” level switch provides control room annunciation in addition to the following:

a. Closes control valve on the incoming cascade drains to HP feeder water heater nos. 5A-5B, deaerating feedwater heater, LP feedwater heater nos. 3,2 or 1.

b. Closes the non-return valve and motor operated isolation valve in the

extraction steam lines to the particular heater concerned with the exception of Heater no. 1, which has no means of extraction steam side isolation.

c. Opens the condensate bypass valves and closes the isolation valves for

LP feedwater heater nos. 1,2 or 3.

d. Opens the feedwater bypass valves and closes the isolation valves for HP feedwater heater strings 5A-6A or 5B-6B, depending on which string is experiencing the high level condition.

Each heater drain control valves, are provided with open/close position indicating lights as well as manual over-riding control switches in the control room. START UP The heater vents and drains system remains in service at all times and would normally be in service throughout the unit start-up. With the loading of the turbine, as the extraction steam flow increases, the heater levels are established. Keep the heater drains in service.

KORBA SIMULATOR

174

During start-up sufficient differential pressure for cascading would not be available in the low pressure heaters. As a result, normal drains would remain ineffective, and heater drains have to be diverted through the emergency drain line. The heaters have to be vented through manually operated shell start-up vent valves during start-up, which are to be closed when steady - state operation has been reached. The heater no.1 has to be vented manually to the condenser during start-up, and this vent will remain open at all times. Shell operating vents of all the heaters would always remain open. STEADY STATE OPERATION The heater drains and vents are designed to operate primarily in the automatic mode over the range of system design loads. Heater levels are maintained by the inherent balancing characteristics of the cascaded system and heater level is maintained by proper positioning of the forward level regulating valves or the bypass valves. Venting is regulated by the orifices in the vent lines to the condenser (or atmosphere in the case of the Deaerator). At low load conditions, wherein sufficient differential pressure for cascading might not exist in the low pressure heaters, the drains will be diverted to the condenser.

KORBA SIMULATOR

175

CONDENSER

AND EVACUATION SYSTEM

KORBA SIMULATOR

176

KORBA SIMULATOR

177

GLAND STEAM CONDENSER SPECIFICATIONS Pressure drop a cross gland steam condenser

Condensate side : 0.21 Kg.cm2

Steam side : 0.025 Kg/cm2

Capacity of gland steam condenser : 0.57 T/hr

No. of Blowers : 2 Nos.

Blower rating : 4.0 KW max.

Gland Steam Condenser

Parameter (Design Temperature) Design Value

Shell side : 350oC

Tube side : 100Oc

Hydraulic test pressure (Shell side) : 60 kg/cm2

Weights

Empty : 1240 kg

Flooded : 1690 kg

Operating : 1465 kg

Vacuum Raising System : 2 Nos of 100% each

Capacity of Vacuum pumps in free dry air at Standard

conditions with pump operating at saturated in take

condition of 25.4 mm Hq abs pressure and sub cooled to

4.17oC below temperature corresponding to absolute

suction pressure

: 50 Nm3/hr.

Capacity as above but absolute suction pressure 50.8 mm

Hq abs in place of 25.4 mm Hqabs

: 85 Nm3/hr.

Discharge pressure : 1.033 Ksc abs.

Cooling water inlet temperature

For 1 above : 12.3oC

For 2 above : 24.5oC

Design TTD (Difference of saturation temperature cooling

water inlet temperature.

:

36oC

KORBA SIMULATOR

178

Blank off pressure with CW temperature 36oC at

Pump suction : 51 mm Hq abs

Ejector suction : 30 mm. Hq. Abs

Minimum suction pressure at pump inlet (allowed) at

36oC

: 65 mm Hq abs

No of stages : One

Pump rated speed : 21.9 m/sec

Power required (at pump)

Maximum : 120 kW

As per condition 1 : 80 kW

As per condition 2 : 92 kW

Sealing water pressure : 0.2 KSC (g)

Cooling water pressure : 6.0 KSC (g)

Volume of condenser steam space to be evacuated : 2120 m3

Pump detail.

Pump model No. : 2BE 1 353 - OBL 4

Suction and discharge size : 200 mm NB

Heat Exchanger

Capacity at 50.8 mm Hq abs and water vapour to saturate

in addition to 7.5oF under cooling

: 85 Nm3/hr.

Inlet pressure : 25.4 mm Hq abs

Power at motor terminal box at 25.4 mm abs : 80 kW

Pump running time to evacuate initial condenser volume : 14 mins.

Motor Standard Continuous rating : 155 kW, 50 Hz, 0.4

kV, 3 phase

Rated speed : 590 rpm

Full load current : 280 Amps

Power factor at rated load : 0.82

Type : Squirrel cage,

induction motor.

KORBA SIMULATOR

179

CO

ND

EN

SE

R C

OO

LIN

G W

AT

ER

SY

ST

EM

KORBA SIMULATOR

180

VACUUM PUMP SPECIFICATION OF VACUUM PUMP NO. OF PUMPS : : 2 Nos. Each of 100% cap.

For each unit of 500 MW.

Capacity of vacuum pumps in free dry air at

standard conditions with pump operating at

standard intake conditions of 25.4 mm of Hg abs.

Pressure and sub cooled to 4.17OC below

temperature corresponding to absolute suction

pressure

: 50 Nm3 /hr.

Capacity as above but absolute suction pressure

50.8 mm of hg abs. In place of 25.4 mm of HG abs

: 85 Nm3 /hr.

Discharge pressure : 1.033 KG/Cm2 abs.

Cooling water inlet temperature

For conditions 2 above : 12.3OC

For conditions 3 above : 24.5OC

Design TTD (Difference of saturation temp - cooling

water inlet temperatur

: 13.9OC

Minimum suction pressure at pump inlet (allowed)

at 36OC cooling water temperature

: 65 mm of Hg abs.

Number of stages : one

Pump rated speed : 590 rpm

Rotor /vane tip speed : 21.9 m/sec.

Power required (at pump)

Maximum : 120 KW

As per condition 2 above : 80 KW

As per condition 3 above : 92 KW

Cooling water required : 75 Nm3 /hr.

Sealing water flow (closed circuit) : 32 Nm3 /hr.

Sealing water pressure : 0.2 KG/Cm2 (g)

Cooling water pressure : 6.0 KG/Cm2 (g)

KORBA SIMULATOR

181

Volume of condenser steam space to be evacuated : 2120 M3.

Pump details.

Pump model No. 2BE 1 353 - OBL 4

Suction and discharge size : 200 mm NB

Heat exchanger

Tube size ODX thickness x length in mm

: 9.5x0.8x3810

Air ejector size : 150 mm

Separator size : 1600 litres.

Guaranteed performance. :

Capacity at 50.8 of Hg abs. And water vapour to

saturate in addition to 7.5OF under cooling

: 85 Nm3 /hr.

Inlet pressure : 25.4 mm of Hg. Abs.

Power at motor terminal box at 25.4 mm abs : 80KW

Pump running time to evacuate initial condenser

volume

: 14 mins.

Motor standard continuous rating :

: 155KW, 50 Hz, 0.4KV, 3

Phase.

Rated speed : 590 rpm

Full load current : 280 amps.

Power factor at rated load : 0.82

Type : Squirrel cage, induction

motor.

DESCRIPTION OF THE VACUUM PUMP ELMO-F Vacuum pumps are the modern discovery for deaerating steam turbine condensers in to-days power stations. The air, which penetrates into the condenser of power station, reduces its efficiency. Vacuum pumps are used

• To extract this air before the condenser is put into operation (hogging operation)

• To evacuate continuously the leakage air which flows into the condenser during holding operation in order to permit good heat transfer and thus optimum condenser pressure is maintained.

KORBA SIMULATOR

182

Three types of vacuum pump systems are most frequently used now a day in power plants.These are

1. Water jet vacuum pumps

2. Steam jet vacuum pumps

3. ELMO-F liquid ring vacuum pumps.

The ELMO-F vacuum pump is popularly known as ‘Energy Saver’ with ELMO-F vacuum pumps, the steam portion which has been drawn in, condenses in the operating water. This results in a very high specific suction capacity at low power demand. Thus liquid ring ELMO-F vacuum pump comes out to be cheapest installation for evacuating the condenser. Because of their high reliability, flexibility and lowest operating cost, ELMO-F liquid vacuum pumps are the obvious choice for our modern power plants. In all 500 MW, the ELMO-F liquid ring vacuum pumps are used to evacuate steam turbine condensers. PRINCIPLES OF OPERATION & DESIGN FEATURES In vacuum pump gas flow enters the compression chamber from both sides(double flow system). The working fluid is normally water but in special cases solvents, acids etc., may be used. The rotor of the vacuum pump is arranged off-centre in the casing. The rotation of the rotor causes the working liquid in the casing to form a ring which rotates with the rotor. The liquid recedes from the hub of the rotor and the gas being pumped is drawn through the suction port. On discharge side, the ring of liquid approaches the hub again and discharges the compressed gas through the discharge port. The operating water is heated by the steam, which has been drawn, in the condensing and by compression process. Part of this water is discharged with the compressed medium and is separated from the air in a separator attached to the pump and led-off. This water must be replaced with fresh and cool water. The cooling of this operating water is undertaken in the heat exchanger. The comparatively cold operating water from heat exchanger is drawn in by the ELMO-F vacuum pump at the rotor hub and thus seals the gap between the non-contact rotor and the flat port plates. The operating liquid supplied at atmospheric pressure and the quantity is controlled automatically internally. Thermal loads, any back pressure and bubble of water on the suction side have no detrimental effect due to the single stage design, the flat port plates, suitable clearances and automatic control of the required operating liquid flow. Single stage ELMO-F liquid ring vacuum pumps have only one moving part, the rotor, which rotates in the casing without contact. There is, therefore, practically no wear during continuous operation and the pumps require little maintenance. The pump material is selected according to the plant conditions.

KORBA SIMULATOR

183

The direct contact between the cooling liquid ring and the water vapour/air mixture being compressed, during holding operations, results in the water vapour condensing in the vacuum pump. This condensation increases the suction capacity of the ELMO-F pump considerably, compared to when dry air is being extracted. Power requirement do not increase. Major studies on ELMO-F vacuum pumps reveal that condensing effect depends substantially on how the operating liquid is supplied and the design of the impeller. The typical vacuum pump casing is elliptical. The rotor is normally sealed with readjustable stuffing box packing and the sealing sections are fitted with replaceable wearing sleeves. The size of the vacuum pump in condenser operation is determined by:

1. The designed leakage air mass flow

2. The required suction pressure

3. The given mixture subcooling.

The air sucking capacity of vacuum pump is a function of suction pressure and the mixture of sub cooling. It is clear that the size of the pump is affected significantly by the sub cooling and the suction pressure for which it is designed. Again leakage air mass flow depends on the condenser steam mass flow at rated load. Hence, size of the vacuum pump is determined after giving due considerations to all these above factors. The air, which penetrates into the condenser of a power station, reduces its efficiency. Before the condenser is put into operation, the air must be extracted (Hogging operation) and also during actual operation (holding operation). The vacuum pump serves this purpose. Vacuum pump works under two modes -

1. Hogging operation

2. Holding operation HOGGING OPERATION In hogging operation both vacuum pumps operate in parallel and air extracted from the condenser into the suction pipe of both the vacuum pump units via the butterfly valve (16 a & 16 b). The butterfly valve (19 a) remains closed. The air ejector is idle.

KORBA SIMULATOR

184

The suction pressure is indicated on the vacuum gauge (1 b). Hogging operation ends when a suction pressure of about 120 mbar abs. is reached. HOLDING OPERATION In holding operation only one vacuum pump remains in operation shutting down of the standby unit. Under special conditions such as

• low partial load

• fouled heat exchanger (13)

• unexpected high pressure loss in the suction piping from condenser to vacuum pump.

The suction capacity can be improved with the air ejector (2) installed upstream.The Vacuum pump can achive a suction pressure of approximately 10mbar withan air ejector with improved capacity. Based on the measured values of the pressure transmitter PT (1Z) and the resistance thermometer TE (45), the delivered ISKAMATIC will decide whether the operation with or without air ejector is favourable. At operation without air ejector (2) the air is extracted from the condenser to the vacuum pump directly via the butterfly valves (16 a and 16 b). At operation with air ejector (2) on signal from Iskamatic the butterfly valve (19 a) will be opened and the bypass valve (16 b) will be closed. Operating air flows to the air ejector (2) via pipe (19) and air from the condenser flows to the vacuum pump via butterfly valve (16 a) and air ejector (2) making the air ejector operative. Together with part of the working fluid, the air is passed via the wet pressure line (17) to the separator (8) where the liquid is mechanically separated form the air. The air leaves the unit at atmospheric pressure via check valve (18 c) at discharge connection. The separated liquid is recirculated to the vacuum pump through a heat exchanger (13) in which the waste heat is removed. The separator is provide with a water level indicator (23), drain valve (24 a), overflow regulator (22b), feed regulator (22 a ) with bypass valve (24 b), and flow meter (20 b) with shut off valve (20 a). To measure air leakages the valve (18 c) must be closed. The air thus flows through flow meter (20 b). Air leakage measurements can only be taken during holding operation. During start up and hogging operation it is important that valve (18 c) is open. The check valve (18 c) is equipped with a hand lever to keep the valve disk closed during air leakage measurement. During air leakage measurement valve (20 a) must be open. The function of the pressure switch (47) is the cutting in/out of the standby - unit.

KORBA SIMULATOR

185

The function of the difference - pressure switch (47 b) is to open the system inlet valve (16 a), when a difference pressure of 30 mbar between condenser and pumpside is reached. The pressure switch (47 a) is installed to cut in/out the air ejector (2) in case of ISKAMATIC failure. By hooking up the fresh water supply with dirt trap (22 h), the feed regulator (22 a) supplies fresh water to the set into the separator and the pump and continues to do so until the desired level is attained. The liquid level in the separator is shown by the water level indicator (23). The overflow regulator (22 b) drains off excess water through the pipe (24). The shut-off valve (24 a) is used for complete draining of the separator (8). The heat exchanger (13) is completely drained through the shut -off valve (24 e) and the pump through shut - off valve (241). The operating liquid is required to make up the liquid ring in the pump. It is drawn out of the separator (8) via pipe (26), then fed to the heat exchanger (13) and after cooling returned to the pump through pipe (28). The heat due to compression and also the heat resulting from condensation of the vapour drawn in with the air are thereby drawn off from operation liquid and dissipated. Line (28) is provide with temperature indicator (42) to monitor the operating liquid temperature and the Vacuum pump is provided with pressure indicator (48) to monitor the operating liquid pressure. The cooling water inlet to the heat exchanger (13) is equipped with a duplex filter (32f). AUTOMATIC CONTROL FOR AIR EJECTOR OPERATION (ISKAMATIC) The automatic control is cutting in/out the air ejector. By monitoring the temperature of the water ring and the suction pressure, the ISKAMATIC will decide whether the operation with or without air ejector is favourable. The typical characteristic line of suction pressure and water ring temperature has been given in the attached sheet. Will the actual suction pressure, recorded by an absolute pressure transmitter fall short of the tolerable pressure at the actual water ring temperature a signal generated by the automatic control will be utilised to cut in the air ejector. The pressure P limit for an operating pressure range without problems will be calculated by means of the recorded water ring temperature. This pressure will be compared with the actual operating pressure PD. PD is increasing the air ejector will be cut off, the moment P limit and a given hysteresis is passed.

KORBA SIMULATOR

186

PD < P limit ejector ON

PD < + P hysteresis ejector OFF

HOGGING AND HOLDING LOGIC

• When both pumps are ON and condenser pressure > 200 mbar then Hogging operation.

• When condenser pressure is < 120 mbar then standby pump stops.

• When standby pump is off then holding operation with one pump.

• When holding operation is going on with one pump and condenser pressure >

200 mbar or running pump has tripped then standby pump takes auto start. MODIFICATION IN THE VACUUM PUMP AT KSTPS In the original package unit, the cooling water for the surface heat exchangers was raw cooling water taken from ARCW pump discharge. As korba is located in the tropical zone, the CW temperature in summer seasons is maintained high. Thus, invariably in summer season, the seal water temperature was maintaining at very high value (45-50 deg. C). In addition to this, seal water temperature problem, the other problems faced due to this raw cooling water were

• frequent choking raw water duplex filters.

• Clogging of heat exchanger tubes. Due to these problems performance of liquid ring vacuum pump was getting deteriorated frequently. Hence, the cooling media for surface heat exchangers was changed to equipment cooling water (DM water) from raw cooling water. The equipment cooling water after being cooled in plate heat exchangers is used for cooling the seal water in heat exchanger of vacuum pump. The ECW being DM water the choking problem is eliminated. The seal water temperature is maintained 3-5OC less as cooling media flow increased due to high pressure (5-7 kg/cm sq.) in ECW system.

KORBA SIMULATOR

187

VA

CU

UM

PU

MP F

LO

W D

IAG

RA

M

KORBA SIMULATOR

188

AUTOMATIC CONTROL OF AIR EJECTOR OPERATION

KORBA SIMULATOR

189

HP AND LP BYPASS SYSTEM

KORBA SIMULATOR

190

KORBA SIMULATOR

191

HP/LP BYPASS SYSTEMS H.P. BYPASS SYSTEM The H.P. Bypass system in co-ordination 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 on Barring Gear .

• Raising of steam parameters to a level acceptable for T.G. 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-out provided boiler load is less

than 60 %.

• Preventing safety valves opening at raised steam pressures. The HP-Bypass system consists of two parallel branches that divert steam from the M.S. line to C.R.H. 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 345OC. The M.S. pressure ahead of the turbine is maintained by two nos. of pressure reducing valves BP-1 and BP-2 combined with valve mounted electro-hydraulic actuator. The steam temperature downstream of the HP - Bypass station is maintained by 2 nos. of spray water temperatures control valves BPE- 1 and BPE -2 combined with valve mounted electro-hydraulic actuators. The spray water is available from the BFP discharge line. there is also one no. spray water pressure control valve combined with the valve mounted electro-hydraulic actuator.

KORBA SIMULATOR

192

HP B

YPA

SS S

YST

EM

KORBA SIMULATOR

193

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 electric motor driven axial-piston oil pump sucks the hydraulic fluid 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 the system with pressurised oil and covers all peak supply requirement. The oil 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 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 motor. From the supply manifold the oil is fed through the pipework and the 3-micron pressure filters to the appropriate control valves and the actuators. SERVOVALVE The two stage servo valve 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 servovalve) and operates the pilot stage (1 st stage), which controls the position of the control piston (2 nd 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 de-energised 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 also local manual deblocking. 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.

KORBA SIMULATOR

194

HP BYPASS: ELECTRO HYDRAULIC SERVO SYSTEM

KORBA SIMULATOR

195

TECHNICAL DATA

Oil Pump OV 16 OV32 UNIT

Supply capacity 12 24 l/min

Speed 1500 1500 Rpm

Motor

(Standard Motor)

Nominal power 4 7.5 KW

Voltage 380 380 V

Frequency 50 50 Hz

Phase 3 3

No load rpm 1500 1500 Rpm

Oil Tank

Oil tank volume 45 70 Litre

Useable volume 20 50 Litre

Hydraulic Accumulator

(Standard)

Nominal volume 10 30 Litre

Pressure rating 200 Bars

approx. Ambient min. 15OC

approx. Ambient max. 65OC

Operating gas Nitrogen only

Bladder material Perbunan (synthetic rubber)

Available Oil Pressure

The controlled system pressure

(set with the pressure reducing

valve)

25 to 120 bar

The max. oil pressure (limited

with the pressure relief Value)

50 to 180 bar

Pressure Switch

4 micro-switches for the set

points

Pump motor - on

Pump motor - off

Pressure too low

Pressure too high

KORBA SIMULATOR

196

Electrical Rating 20 Amp at 488 V AC

10 Amp at 125 V AC

0.25 Amp at 250 V AC

0.5 Amp at 125 V AC

MODE OF OPERATION HP Bypass system is intended to ensure reheater protection, minimum superheater 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. The 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 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 R.H. 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 boiler firing rate will be maintained at the 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. 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 pipings.

KORBA SIMULATOR

197

CONTROLS AND INTERLOCKS 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. TEMPERATURE CONTROL The control positioners for the Bypass spray temperature valves are designed in the same way as those for the HP Bypass valves. In addition PI controllers are also connected upto the control positioners. The temperature measuring signal from transmitters is compared at the PI controllers with the common temperature set point. According to particular control deviation the PI controller forms a rated signal for the control positioners of the associated temperature valves. The electro-hydraulic actuators make it possible to attain short positioning time for the spray water temperature control valves and then allow the temperature control to intervene fast enough in the event of quick opening of the HP Bypass valves. To off-set the time delay of temperature measurement and to achieve favourable conditions when reaching on the spray water cooling system (rapid adjustment to temperature input of the injection value 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 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 feed water pressure regulating valve BD. TEMPERATURE CONTROLLER (BPE VALVES) & POSITIONING LOOPS The purpose of this controller is to reduce the HP Bypass steam temperature by injecting water into the BP valves, in accordance with the reheater inlet temperature. PI action controllers drive hydraulic actuators to move the valves to maintain the steam temperature downstream of the BP valve at the preselected value. The temperature deviation, after passing through PI controller produces the valve position demand signal, which drives the proportional controller of the positioning loop, unit it is balanced by the BPE valve position feed back until it is balanced by the BPE valve position feed back signal, from the position transmitter fitted on the actuator.

KORBA SIMULATOR

198

OVERRIDES AND INTERLOCKS BPE OPENING SIGNALS Two anticipatory signals are given to the P part of the PI-P temperature controller. The first one is proportional to the BP valve position, the other one is a fixed value which is given as soon as the fast opening device of the corresponding BP valve is activated. This will ensure immediate opening of the spray valve as the BP valve opens, to counteract the measurement lag associated with temperature measurement. BPE AUTO INTERLOCK As the BP valves start to open, an auto signal is given to the BPE positioning loops. But the controllers of these loops are put on Auto only when the actual temperature is greater than an adjusted value. THERMOCOUPLE BREAKAGE If the thermocouple breakage is detected in the transmitter, an alarm is given and the BP control will be put on manual. TEMPERATURE HIGH An alarm is given when the attemperated steam temperature rises above a pre-set limit. TEMPERATURE TOO HIGH If the temperature rises still further and exceeds the set value, a closing override will be given to the BP control, also putting it on manual, to protect the condenser. SPRAY WATER PRESSURE CONTROLLER, BD VALVE The purpose of this controller is two fold. First, it serves to maintain the spray water pressure at a constant value to achieve favourable conditions for injection. Secondly, the BD valve serves as an isolating valve when the BP valves are closed to eliminate any dribbling of spray water into the BP valves. The downstream pressure signal is compared at the input of the controller with the “desired value” signal coming from the set point formation. The resultant control deviation is fed to a PI controller which produces valve position demand signal. This signal drives the summing amplifier of the positioning loop until it is balanced by the BD valve position feedback signal coming from the position transmitter.

KORBA SIMULATOR

199

OVERRIDES AND INTERLOCKS CLOSING OVERRIDES : this override is given when the BP valves are closed (below 2 %) and the PI output is less than 2 %. This is to prevent any spray water from entering the system. AUTO INTERLOCK : When the BD valve closing override is removed, a signal is sent to put the controller on Auto. This is to ensure correct attemperation when BP valves start opening. OPENING SIGNALS : Anticipation signals proportional to BP valve positions are given to P part of the PI-P controller. This is to ensure immediate opening of the BD valve when BP valves start opening. So it can be summarised as follows : 1. HP Bypass valve BP-1 opening less than 2 % will automatically close the spray water pressure control valve (BD valve). 2. It 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 & BPE 2 shall be changed to ‘AUTO’ mode irrespective of their initial conditions. 3. If BP valves position drops 2 % open, it will receive an auto close command to ensure positive shut-off. 4. If the steam temperature downstream of the BP valves becomes 380 deg C, 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 :- 1. Generator breaker open.

2. Turbine load shedding relay operated

3. Pressure controller deviation more (+) 10 %.

4. Depressing of the ‘FAST OPEN’ push button from UGB.

L.P. 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

KORBA SIMULATOR

200

Bypass valves are two in number. the LP Bypass stop and control valves are combined in a common body. 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 desuperheating purpose. This injection water is taken from the discharge of the condensate extraction pumps.

LP BYPASS SYSTEM

THE LP TURBINE BYPASS CONTROLLER (LPB) LPB COMPRISES OF :

• Pressure control loop.

• Valve position control loop.

• Tracking unit.

• Actuation of reheat safety valves.

• Automatic control interface.

• Condenser temperature protection (CTP)

KORBA SIMULATOR

201

PRESSURE CONTROL LOOP The pressure control loop consists of :

1. Set point derivation equipment. 2. Pressure controller. 3. Actual pressure derivation equipment.

Two pressure setpoints are derived for the LPB and are gated in an auctioneer. One is the fixed set point and the other is the variable setpoint. The variable setpoint is obtained with the aid of a pressure transducer upstream of the HP blading (referred to as wheel chamber pressure). This provides a load dependent setpoint and hence reflects the dependence of the actual pressure signal on steam flow. The variable setpoint is limited to an upper value by an adjustable limiting function which is kept well below the response pressure of the reheat safety values. During start-up and shut-down the variable setpoint is suppressed by a fixed setpoint. The fixed setpoint can be adjusted between 0 and 120 % of the maximum reheat pressure from the control room. Actual pressure is obtained by means of a pressure transducer in the reheater outlet (hot reheat line). A PI action pressure controller acts on the deviation between the actual pressure and the higher of the variable and fixed setpoint signals. VALVE POSITION CONTROL LOOP The pressure controller output signal acts as the setpoint signal for the connected valve lift controller. The valve lift controller acts as a slave controller for the pressure control loop. This subordination improves both the stability and the dynamic response of the control system as a whole. The input signal for the valve lift controller is the deviation between the actual valve lift and the valve lift setpoint received from the pressure controller, the lift of the LP turbine Bypass control valves is governed by the position of the servopistion in the electrohydrualic converter. The spray valves and the LP Bypass valves are actuated in accordance with pre-set characteristics. The LPB can be transferred in the control room from governing to manual control by depressing the “controller on/off” push button. It is thus possible to adjust the valves directly by depressing the OPEN and CLOSE push buttons. There is also automatic transfer from governing to manual control during certain fault conditions to prevent incorrect control actions.

KORBA SIMULATOR

202

LP BYPASS CONTROLLER

TRACKING UNIT Continuous tracking of the controlling variable which is not in action is provided to ensure bumpless transfer between control system and manual at all times. In the “automatic governing” mode, the manual setpoint adjuster is automatically tracked to the controller output signal. In the “manual mode” the valve lift controller signal is automatically tracked to the manual setpoint. However, zero deviation between the pressure setpoint and the actual pressure would be required for bumpless transfer from manual control to automatic governing. If the transfer is made in spite of an existing control deviation, this is compensated subsequently by the controller, which repositions the control valves as appropriate.

KORBA SIMULATOR

203

LPBP EHC POSITION Vs VARIOUS VALVE OPENING AUTOMATIC CONTROL INTERFACE This acts as a centralised control for the proper operation of the LP Bypass controller. When ACI is switched on, the fixed pressure setpoint is set to a value of approx. +3 bar above the actual pressure as soon as the “Light up” signal is given at the start up sequence. A minimum aperture of 25 % is applied which causes the desuperheating spray, Bypass stop and control valves to be opened during start-up. This is to ensure minimum flow through reheater. To achieve a rapid pressure build-up, the Bypass valves are retained at this aperture till the actual reheat pressure crosses + 12 bars. This is referred to as “Hold process”. In this process the fixed setpoint is automatically tracked to the actual pressure (tracking mode). Control is transferred to automatic governing only when the reheat pressure is above + 12 bars. The fixed setpoint is thus maintained at + 12 bars. When a Bypass valve lift of approx. 35 % is reached, the ACI for the fixed setpoint is switched off. The variable setpoint takes over from the fixed setpoint through the auctioneer and thus governs the reheat pressure setpoint. The fixed set point of +12 bar is reached at unit shut down also. In this manner, sufficient flow through

KORBA SIMULATOR

204

reheater is ensured at all time, as also optimum raising of the reheat pressure is achieved.

LP BYPASS CONTROL SYSTEM

CONDENSER TEMPERATURE PROTECTION The purpose of the condenser temperature protection is to protect the condenser from excessively high steam inlet temperatures.Thermocouples output temperature signals are passed to an interlock circuit which locks out the LP turbine Bypass station. Protective Closing Of Bypass System (Condenser Back-Up Protection) The LP Bypass valves will close automatically under the following normal conditions to prevent damage to the condenser.

1. If the steam pressure downstream of LP Bypass valves is greater than 19 kg/cm2.

2. Condenser vacuum is low (0.4 kg/cm2 abs)

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

4. Condenser wall temperature is high (90OC).

KORBA SIMULATOR

205

High exhaust hood temperature will automatically switch on the exhaust hood spray water. In case of condenser wall temperature protection operating, the ‘RESET Bypass TRIP’ - RB, for solenoids SV-1 and SV-2 have 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 (EHG) 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 feed - back mechanism stabilises the amplifier piston for a given voltage change. The sleeves (KA04) of follow-up piston valves (KA02/KA03) also move down increasing the signal oil pressure to slide valve of water injection valves (MAN 11+12 AA003), thereby opening them, in the beginning of control operation.

1. Electric LP bypass governor 2. Plunger coil measuring system 3. Jet pipe 4. Adjusting spring 5. Adjusting screw 6. 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 stop valves (MAN 11+12 AA001) 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). LP Bypass control valves (MAN 11 + 12 AA002) open up due to hydraulic feed back between actuator pistons and pilot values . 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 LPSVs and LPCVs to open. In case of condensate water pressure low and condenser pressure high the reverse action takes place and the spring of KA02 is detensioned to such an extent that LP Bypass valves are unable to open.

KORBA SIMULATOR

206

1. Jet pipe 2. Jet pipe regulator 3. Adjusting spring 4. Adjusting screw 5. Corrugated measuring system 6. Adjusting spring 7. Corrugated measuring 8. Corrugated measuring 9. 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

lI. Vacuum signal from bypass steam piping behind bypass control valve

LP BYAPSS LIMITING REGULATOR

PROTECTION DEVICES LOW VACUUM SAFETY DEVICE A low vacuum safety device (MAG 01 AA016) is installed on the signal oil line from follow-up piston KA02 to Bypass valve’s pilots and if vacuum drops below a pre-set value; the valve of the safety device moves downwards due to increasing pressure above it. The valve thus block off the signal oil thereby closing the LP Bypass stop and control valves. As vacuum increases, Bypass operation is restored in reverse sequence when the pre-set vacuum has built up. LOW INJECTION WATER PRESSURE A pressure switch (MAN 01AA011) is installed in the signal oil line from KA02 to spool valves KA02 and KA05 of LPBypass valves, to protect the condenser in the event of water injection failing. If the injection water pressure drops below a pre-set value, the valve of the pressure switch (MAN 01AA011) moves down, blocking off the signal oil line and depressuring the oil thereby closing the LP Bypass valves due to low condensate water pressure. Bypass operation is restored in the reverse sequence when injection water pressure becomes normal. HIGH CONDENSER WALL TEMPERATURE At a pre-set condenser wall temperature the two thermocouples mounted in steam dome opposite to bypass steam inlet transmit a switching pulse to the associated solenoid valves (MAX53AA021+022). The solenoid values block off the depressive

KORBA SIMULATOR

207

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 one after the solenoids are manually reset after the temperature become normal. TWO STAGE WATER INJECTION To prevent undue overloading of condensate pumps under normal shut-down/start-up conditions, the injection water demanded from CEPs is staggered in two stages. This arrangement opens the injection valves (MAN11+12 AA004) via the pressure switch (MAN01CP001), solenoid valve (MAX53AA041) & slide valve when the steam pressure upstream at the expansion orifice exceeds value corresponding to 45 % of maximum Bypass flow.

1. Capnut 2. Adjusting csrew 3. Cover 4. Compression spring 5. Diaphragm 6. Valve 7. Valve sleeve 8. Casting 9. Can 10. Lever

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

LOW VACUUM SAFETY DEVICE

KORBA SIMULATOR

208

1. Hood 2. Bellos 3. Pushrod 4. Knife edge lever 5. Cam shaft 6. Compression spring 7. Fitted key 8. Shaft 9. Scale 10. Cylindrical pin 11. Nozzle 12. Slide valve 13. Valve bushing 14. Compression spring 15. Bearing bushing 16. Torsion spring 17. 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

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

HIGH CONDENSER WALL TEMPERATURE

KORBA SIMULATOR

209

L.P. BYPASS CONTROL SYSTEM - MODE OF OPERATION

a) The electro-hydraulic L.P. Bypass controller (proportional action) controls the plunger-coil arranged on the right side of the converter.

b) On an increase in voltage, the jet pipe is deflected to the left (as shown) and the

piston of the actuator is moved downwards.

c) The sleeves of the follow-up pistons connected to the actuator move downwards causing the oil pressure in the following piston to rise.

d) At the beginning of the opening (control) sequence, the rising pressure in the

follow up piston opens the injection water valves via the pilot valves (iii) and the actuators for the water injection valves.

e) The injection water reaches the expansion orifice and is available for cooling the

Bypass steam flowing to the condenser.

f) After a short delay, the L.P. Bypass stop valves also open fully when the oil pressure in the follow-up pistons rises provided the piston of the limit pressure controller is in the upper end position.

g) Next the control valves open to a position depending on the oil pressure as

determined by the feed back between servo pistons and pilot valve. Following points are to be kept in mind before charging LP bypass.

1. Condenser Vacuum should be > -0.7 kg/cm2

2. Spray water pressure > 25 Kg/cm2 3. Temperature solenoids should be in reset condition given on the turbine

console. Remember, if any of the above conditions is not present during LP bypass operation, trip close command will be issued for LP bypass.

KORBA SIMULATOR

210

KORBA SIMULATOR

211

STEAM TURBINE AND

AUXILIARIES

KORBA SIMULATOR

212

KORBA SIMULATOR

213

KORBA SIMULATOR

214

TURBINE AND AUXILIARIES TURBINE SPECIFICATION OF MAIN TURBINE Make : KRAFTWERK UNION, WEST GERMANY

Type : Three Cylinder, reheat, Condensing turbine

No of stages : HP 18 Nos. IP 14x2 Nos. LP 6x2 Nos.

Nominal rating : 500 MW

Peak Loading : 536.7 MW

Rated Speed : 3000 rpm

Max/Min Speed : 3090/2850 rpm

Speed exclusion range : 400 to 2850 rpm.

STEAM PRESSURES & TEMPERATURE (RATED VALUES)

: Pressure (at) kg/cm2 Temperature oC

Initial steam : 170 537

First Stage Pressure : 151.79 537

HP cylinder exhaust : 45 342.5

IP stop valve inlet : 40.5 537

Extraction 6 : 45 342.5

Extraction 5 : 19.52 428.3

Extraction 4 : 7.57 302

Extraction 3 : 2.76 197.8

Extraction 2 : 1.42 188.8

Extraction 1 : 0.286 67.6

L.P. Cylinder Exhaust : 0. 0884 43.1

WEIGHT (TONNES)

: HP IP LP

Rotor : 11.6 21.8 84.6

Cylinder Assembled : 80.0 32.5 86.0

Main stop & control valve : 10

Reheat stop & control

valve

: 17

KORBA SIMULATOR

215

MOMENT OF INERTIA (KG-M2)

Rotor of HP cylinder : 713.0

Rotor of IP cylinder : 2145.6

Rotor of LP cylinder : 22981.0

LIMITING VALUES CASING TEMPERATURE (0C)

Alarm Machine must be shutdown at

HP Turbine Exhaust : 480 oC 500 oC

Outer Casing of LP

cylinder

: 90 oC 110 oC

(SPRAY WATER TO LP CYLINDER MUST BE SWITCHED ON AT 900C)

TEMPERATURE DIFFERENCES (0C) (BETWEEN UPPER AND LOWER CASING

SECTION)

Alarm Machine must be shutdown at

HP Turbine Middle : + 30 oC + 45 oC

I.P. Turbine Front : + 30 oC + 45 oC

L.P. Turbine Rear : + 30 oC + 45 oC

STEAM PURITY (KWU RECOMMENDED VALUES)

Conductivity at 35oC

Alkaline method

: < 0.15 ms/cm

Silica Acid (SiQ2) : < 0.010 mg/kg

Total iron (Fe) : <0.005 mg/kg

Total COpper : < 0.010 mg/kg

Sodium (Na) : < 0.02 mg/kg

BEARING TEMPERATURES (OC)

Alarm M/C must be shut down at.

Normal operating

temperature below 75oC

: 90oC 120oC

Normal operating

temperature above 75oC

: 100oC 120oC

KORBA SIMULATOR

216

VIBRATION (ABSOLUTE VIBRATION)

Bearing Housing Shaft

Nominal Value for alarm : - 30 microns above normal level

Limit value for tripping : 35 mic. 120 microns

Limit value for tripping

manual

: 45 mic. 200 microns.

DIFFERENTIAL EXPANSIONS

H.P. Turbine : + 5mm - 3mm

I.P. Turbine : + 8mm - 2mm

L.P. Turbine : + 30mm - 3mm

MATERIAL OF CONSTRUCTION

CASING

H.P. Outer Casing/Barrel

Casing

: GS 17 Cr MoV 515

H.P Blade carrier : GS 17 Cr MoV 515

I.P. Casing : GS 22 MO 4

L.P. Casing : Outer St. 37 - 2N

Inner I-GS 22 MO 4, H II, 15 MO3

SHAFTS

H.P. Shaft : 28 Cr. MoNi 59

I.P. Shaft : 30 Cr. MoNiV 511

L.P. Shaft : 26 Ni Cr. MoV 145

MOVING BLADES

H.P. Turbine first stage : x 22 Cr. MoV 121

H.P. Turbine other stages : x 20 Cr. 13/x22 Cr. MoV 121

I.P. Turbine stages : x 20 Cr. MO 13/ x 20 Cr 30

L. P. Turbine stages : x 20 Cr. 13

FIXED BLADE

H.P. Turbine first stage : x 22 Cr MoV 121

H.P. Turbine other stages : x 22 Cr MoV 121/x 20 Cr. MO 13

I.P. : x 20 Cr 13/x20 Cr MO 13

L.P : x 20 Cr 13/x 20 Cr MO 1320Mn5/

x 7 Cr MO 13

KORBA SIMULATOR

217

CASING JOINT BOLTS

H.P. Cylindder : 21 Cr MoV 57

IP : 21 Cr MoV 57

L.P : St.50/24 Cr. Mo 5

STEAM TURBINE - DESCRIPTION GENERAL 500 MW Turbine at NTPC singrauli is of Kraftwork Union, West Germany design. The

turbine is 3 (three) cylinder, reheat, condensing tandem compound, horizontal and single shaft machine. It has got 3 cylinders such as high pressure(HP), intermediate

(IP) and low pressure (LP) parts. The HP is a single cylinder 18 stages turbine and IP and LP are double flow cylinders having 14 X 2 and 6 X 2 number of stages

respectively. The turbine rotors are rigidly coupled with each other and with generator rotor. HP turbine has throttle control. The steam is admitted through 4 combined stop and control valves and these stop and control valves are 5/6 meters away from turbine. The lines leading from HP turbine exhaust to reheater have got two cold reheat swing check NRVs. The steam from reheater is admitted to IP turbine through 4 combined Stop and Control valves. Two cross-over pipes connect IP and LP cylinder. BLADING The blading of the HP and IP turbine consists of several drum stages and all stages

having 50 % reaction. The moving blades of HP,IP and front rows of LP have inverted T roots and are shrouded. The last stages of LP turbine are twisted; drop forged

moving blades with fit-tree roots. Highly stressed guide and moving blades of HP and IP are provided with T-root which determine the distance between the blades.

BEARINGS The TG limit is mounted on seven (7) bearings. HPT rotor is mounted on two bearings, a double wedged journal bearing at the front and combined journal and thrust bearing adjacent to front IP rotor coupling. IPT and LPT rotors have self-adjusting circular journal bearings. The bearing pedestals of LP are fixed on base plates where as HP front and rear-bearing pedestals are free to move axially.

KORBA SIMULATOR

218

HP TURBINE The HP turbine casing is designed as a barrel-type casing without axial joint. An axially split guide blade carrier is arranged in the barrel-type casing. The barrel type casing remains constant in shape because of rotational symmetry and leak proof during quick changes in temperature (e.g. on start up and shut down, on load change and under high pressure. The space between timer and outer casings is filled with the main steam.

a. TURBINE ROTOR b. OUTER SEAL RING c. BARREL TYPE CASING

d. GUIDE BLADE CARRIER e. THREADED RING f. CASING COVER

IP TURBINE The IP turbine is split horizontally and is of double shell and double flow construction. Steam from the HP turbine enters the inner casing from top and bottom through two inlet nozzles flanged to the mid section of the outer casing. This arrangement provides

KORBA SIMULATOR

219

opposed double flow in the two blade sections and compensate axial thrust. The centre flow prevents the steam inlet temperature

IP TURBINE

1. Turbine Rotor

2. Outer Casing Upper Part

3. Outer Casing Lower Part

4. Inner Casing Upper Part

5. Inner Casing Lower Part

6. Extraction Zone

7. Inlet Nozzle

from affecting the support brackets and bearing sections. The inner casing arrangement means that the steam inlet conditions are limited to the inlet section of the inner casing, where as the joint of the outer casing is only subjected to the lower pressure and lower temperature prevailing at the outlet of the inner casing. The joint

KORBA SIMULATOR

220

flange can thus be kept small and material accumulations reduced to a minimum in the area of the flange. In this way difficulties arising from deformation of a casing with flap joint due to non-uniform temperature rises e.g. on start-up and shut-down, are avoided. LP TURBINE:

The LP turbine is of double flow and triple shell welded casing. The outer casing consists of the front and rear walls, the two lateral longitudinal support beams and the upper part. The front and rear walls, as well as the correction areas of the upper part one reinforced by means of the longitudinal beams on the base plates of the foundation, steam admitted to the LP turbine from IP turbine flows into the inner casing from both sides through steam inlet nozzles before the LP blading. Expansion joints are installed in the steam piping to prevent any undesirable deformation of the casings due to thermal expansion of the steam piping. The inner casing of LP turbine is a double shell construction and consists of the outer port and the inner port. The inner shell is suspended in the outer shell to allow thermal movement and carries the front guide blade rows. The rear guide blade rows of the LPT stage are bolted to the outer shell of the inner casing. The complete inner casing is supported in the LPT outer casing in a manner permitting free radial expansion, concentric with shaft, and axial expansion from a fixed points on LP turbine. FIXED POINTS - CASING AND ROTOR EXPANSION In designing the supports for the turbine on the foundation, attention has been given to the expansion and contraction of the machine during thermal cycling. The fixed points of the turbine casing on the foundation are as follows: The bearing housing between the IP and LP turbines. From this point the IP and HP casings expand towards the front bearing housing of the HP turbine. LP turbine The rear bearing housing of LP turbine. The front baseplate 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 in the rear bearing casing of HP turbine

KORBA SIMULATOR

221

CASING EXPANSION The front bearing housing of the HP and LP turbines can slide on their base plates in an axial direction. Any lateral movement perpendicular to the machine axis is prevented by fitted keys. The bearing housings are connected to the HP and IP turbine casing by guides which ensure that the turbine casings maintain in their central position while at the same time allowing axial movement. Thus the origin of the cumulative expansion of the casings is at the front bearing housing of the LP turbine. The separate casings of the LP turbine are located axially by fitted keys at the front supports of their longitudinal beam members on the baseplates. Free lateral

LP TURBINE

1. Outer Casing Upper Part

2. Diffusor, Upper Half

3. Outer Shell, Upper Half

4. Inner Shell Upper Half

5. Inner Shell Lower Half

6. Outer Shell Lower Half

7. Diffusor Lower Half

8. Outer Casing Lower Half

KORBA SIMULATOR

222

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

TURBINE ANCHOR POINTS AND EXPANSIONS

1. HP Front Pedestal

2. HP Rear Pedestal

3. LP Front Pedestal

4. LP Rear Pedestal

5. HPT Outer Casing

6. IPT Outer Casing

7. LPT Outer Casing

8. HP Front Pedestal Base Plate

9. HP Rear Pedestal Base Plate

10. LP Front Pedestal Anchor Point

11. LP Rear Pedestal Anchor Point

12. LP Outer Casing Anchor Point

13. HPT Inner Casing

14. IPT Inner Casing

15. LP Inner Outer Casing

16. LP Inner-Inner Casing

17. HP Inner Casing Anchor Point

18. IP Inner Casing Anchor Point

19. LP Inner Outer Casing Anchor

Point

20. LP Inner-Inner Casing Anchor

Point

KORBA SIMULATOR

223

expansion between the outer casing and the fixed bearing housings to which the housings for the shaft glands are attached are taken by bellows type expansion joints.

EXPANSION OF TURBINE CASING

ROTOR EXPANSION The thrust bearing is incorporated in the front bearing housing of the IP turbine. Since this bearing housing is free to slide on the baseplate the shafting system moves with it. Since 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.

EXPANSION OF TURBINE ROTORS

DIFFERENTIAL EXPANSION Differential expansion between the rotors and casing results from the difference between the rotor expansion originating from the thrust bearing and the casing expansion originating from the rear bearing housing of IP turbine (Bearing No.3

KORBA SIMULATOR

224

housing). This means that the maximum differential expansion of the HP and IP turbine occurs at the end furthest from the thrust bearing. Differential expansion the rotors and casing of the LP turbine results from the difference between the expansion of the shafting system originating from the thrust bearing and the casing expansion originating from the fixed point point of LP turbine casing on the longitudinal girder. When the steam turbine is running relative displacement occurs between the rotor of the turbine and their casing due to their different thermal inertia. This is the case particularly during start-up and in case of major load changes. These measurements are monitored continuously by contactless measurements. The figures below elaborate the location of sensors for measurement differential expansion HP, IP & LP turbine, which are fitted in front pedestal, pedestal No.3 and pedestal No.4 respectively.

CASING AND ROTOR EXPANSION

LOCATION OF DIFFERENTIAL EXPANSION SENSORS OF HP, IP & LP TURBINE

ABSOLUTE EXPANSION Measurement of absolute expansion is carried out both at the front pedestal as well as at the middle-bearing pedestal. The displacement senser is mounted on the foundation and connected to the casing via a rope. The other end of the rope is connected to a rotatable measuring device, which forms magnetic return path for the two sensor coils.

KORBA SIMULATOR

225

The rotating measuring device rotates eccentric to the two sensor coils, which creates different air gaps and thereby different inductances. Thus the air gap between measuring device and the coil depend on the angle of rotation BLADING In steam turbines, the blades transform the thermal energy into mechanical energy, it is obvious that blading has got direct impact on the efficiency and reliability of turbine. Appropriate blade profile, with high aero-thermodynamic efficiency having sufficient mechanical strength to with stand the steam forces, are determined after extensive research. Particular care is taken to avoid resonance and to ensure that the natural frequencies of unshrounded free stand rotor blades are compatible with the rotational frequency of the machine. A final accurate check is made when the blades have been fitted into the rotor. The opportunity is also taken to carefully check the root fastening of the blades. HP & IP BLADING These blades have a 50 % reaction component and both fixed and moving blades have the same profile. Each rotor blade is milled from a single piece of material complete with inverted T-root and integral shroud. After they have been fitted in the rotor grooves, they are caulked into position with brass wire. The blade roots are made of appropriate size to give the required blade spacing, and hence the designed width of blade passages, without the necessity for spacer-pieces. Fixed blades are of the same type as the moving blades with an inverted T-root and integral shroud. NON-TWISTED BLANDES FOR THE LP TURBINE. The fixed blades are of the same type as the moving blades with an inverted T-root and integral shroud. TWISTED BLADES FOR THE IP TURBINE. The last four stages of the LP turbine have twisted blades. The difference between the circumferential velocity at the rotor blade root and tip is quite considerable and is taken into account by twisting the blade along its length. The rotor blade fastening is of the fir-tree type which engages in milled axial grooves in the rotor and is then secured. The fixed blades of the last two stages are hollow. They are fabricated from sheet steel and slots are provided in the blade surface through which any water passing over the surface of the blades may be drawn away to the condenser. The trailing edge of the blades is very thin inorder to avoid any stall patches and the formation of stream of water. The axial distance between the final stages is kept at

KORBA SIMULATOR

226

optimum value to facilitate the break-up of any water droplets which may still remain. This will reduce the relative velocity between the drop-lets and the leading edges of the moving blades. The leading edges of the final stage rotor blades are flame hardened to give protection against erosion. The blades are free-standing and have neither lacing wires nor shrouding. Great attention is paid at the design stage to achieve a large resonance free range of operation. BLADE TIP SEALING In reaction turbine stages, there is a drop in pressure across both the fixed and moving blades. This pressure differential between the inlet and outlet sides of the blades also causes steam to flow over the tips of the fixed and moving blades. This amount is a loss and inorder to keep it as small as possible it is essential to provide proper sealing at this point. Thus, both fixed and moving blades have continuous shrouding in which steps have been turned to produce a labyrinth seal. Seal strips from the fixed or moving components project into the steps turned on the shrouding. The complete rings of shrouding on all rotor blades, and on guide blades are built up from the individual sections of shrouding which are machined integrally with each blade fitted to butt tightly together. When the rotors and guide blade carriers have been completely built, the blading is skimmed on a lathe, the steps being machined on integral shrouding. Various arrangements of tip sealing employing three or four rows of sealing strips are used. The number of rows of sealings strips chosen depends on the stage pressure and the differential axial expansion between the rotor and the casing at the particulars section of blading involved. The setting strips which are caulked into the casing and shaft opposite the blade shrouding are of stainless steel. On one hand they are strong enough to with stand the maximum pressure which will exist across them and on the other hand, the amount of heat generated by them and transmitted to the rotor or casing in the event of their rubbing shall not be sufficient to cause deformation of the components. The sealing strips are easy to replace. If contact occurs between the fixed and moving components at any time and the sealing strips wear out, the new ones could be fitted within a short time to restore correct clearances at the next overhaul. SHAFT GLANDS Labyrinth-type glands seal the shaft where it passes through the casing. In the case of HP and IP turbine, these consists of a series of sealing strips alternatively caulked

KORBA SIMULATOR

227

into the shaft and into the stationary rings. In the case of the LP turbine, sealing strips are fitted in the stationary rings only. The pressure of the steam leaking through the gland is reduced by converting the pressure energy into velocity energy, which is then dissipated as turbulence as the steam passes through large number of strips. The number of sealing strip rings used depends on the pressure drop required. Each ring consists of six or eight segments and is carried in grooves in the casing or inner casing to allow radial movement. Each segment is held in position against a shoulder by two coil springs so that a fixed reference for the clearance of the shaft is provided. In the event of the shaft coming into contact with the strips in the segments, the affected segments will move away radially. A possible slight contact between the shaft and the thin strips will generate only a small amount of heat which cannot lead to deformation. The turbine glands are self-seal type beyond approx. 40 % load, for initial sealing purpose steam from auxiliary source through the gland steam valve is taken to seal all the HP, IP & LP glands. During this period the leakage steam valve connecting this header to condenser is kept closed. After approx. 40 % load, gland steam valve is closed and leakage steam valve is opened. Pipings are so sized that the leak off steam from front and rear end of HP turbine goes to the condenser through the leakage steam valve, while steam from the front and rear ends of IP glands is utilised for sealing the LP glands, thus proper temperature matching is ensured. The leakage of steam and air from the last chambers of each rotor is sucked into a gland steam condenser. BEARING COUPLING AND TURBINE GEAR In 500 MW turbine generator consists of 3 cylinders e.g. High Pressure, intermediate pressure and low-pressure turbine, one generator and one exciter and all are coupled to each other by solid couplings. The rotating part of each component is supported in bearings and the axial position of the shaft is determined by a thurst bearing and there is only one thrust bearing, which determines the entire axial position of the turbine generator. If flexible couplings were used then a thurst bearing was required for each loosely connected shaft. JOURNAL BEARINGS The function of the journal bearing is to support the turbine rotor. Essentially, the journal bearing consists of the upper and lower shells, bearing cap spherical block, sperical seat and the keys. The bearing shells are provided with a babbit face. The sliding surfaces of this bearing are machined and additional scrapping is neither necessary nor permissible. Both bearing shells are fixed by means of taper pins and bolted together. White metal journal bearings are used because of high loading capacity, reliability and the absence of wear. The long life of the bearing is due to the

KORBA SIMULATOR

228

fact that when the shaft is rotating at speed, a continuous high pressure wedge of oil is automatically formed between the white metal and the shaft. The presence of this oil ensures that no metallic contact takes place and consequently no wear occurs. The continuous supply of lubricating oil is must to flush the bearing remove frictional and conducted heat; the majority of the oil is used for this purpose. Annular recesses at each side of the bearing collect the side leakage of oil from these recesses the oil drains to the pedestal and flows out through the sight glass boxes. Oil guards and oil throwers prevent oil from passing along the shaft and reaching the glands. High pressure jacking oil is admitted through small holes at the bottom of the bearing over come break away torque and prevent dry friction, to lift the shaft when starting from rest, this not only enables our oil film be established and so prevent wear when starting but it also reduces the starting torque on the barring motor. THRUST BEARING It provides a positive location for the rotor relative to the casings (bearing no. 2 is only thrust bearing) and with stands the unbalanced thrust caused by blade reaction and steam pressure acting on unbalanced areas. COUPLING There is limit to the length of the individual cylinders forming the complete turbine. At the same time there is a limit to the length of turbine shaft that can economically be made. So there is need for couplings to join the individual cylinders (rotors) together to transmit the driving torque to each other and the generator rotor. On large turbines, the high torque is to be transmitted so the use of flexible couplings become impractical. So rigid couplings are used between the turbine shafts, so that the entire shaft behaves as one continuous rotor. The use of solid couplings means that only one thurst bearing is not practicable, since the entire shaft expands away from the thurst bearing. In 500 MW turbine thurst bearing is situated between the high pressure and intermediate pressure cylinders, in this way the differential expansion of the two cylinders and rotors is minimised. This in turn has led to the practice of reversing the high pressure cylinder so that the steam flow away from the thrust bearing, in this way the differential expansion at the inlet end of the cylinder is minimised. SHAFT ALIGNMENT A long shaft bends naturally under its own weight. It revolves about its curved centre line and because of this the positioning of the bearings is vital to ensure that the shaft runs smoothly, since the natural curve of the shaft must be maintained at all times, with the aid of coupling checks it is possible to align the journal bearings in such a way as to make the entire shaft assembly follow the continuous deflection curve and

KORBA SIMULATOR

229

this measurement is performed by using dial gauges, micro meters etc. TURNING GEAR Turning Gear is provided to rotate turbines shafts slowly during the pre run up operation and after shut down, to prevent uneven heating or cooling of the shafts; the uneven heating or cooling would lead to bending between the fixed and moving with possible fouling between the fixed and moving parts. The turning gear spread is chosen to ensure satisfactory lubrication of the bearings and, at the same time, provide some measure of circulation of the air with in the casings after shut down particularly at the low pressure end, so contributing to their uniform cooling. Another advantage of training gear is that the necessity for suddenly admitting a large flow of steam, in order to start the turbine from rest is avoided, and this therefore prevents server temperature gradients occurring. It is therefore of prime importance that whenever a turbine generator is to be started or while it is hot after being shut down, the turning gear should be in service. In 500 MW machines hydraulic turning gear system is provided backed up with manual turning gear. HYDRAULIC TURNING GEAR During turning gear operation, the shaft system is rotated by a double row blade wheel which is driven by oil provided by the auxiliary oil pump. This oil passes via a check valve into the nozzle box and then into the nozzles which conduct the oil jet in front of the blading. After passing the blading, the oil drains into the bearing pedestal and flows with the bearing oil into the return flow piping. MANUAL TURNING GEAR A manual turning gear is provided in addition to the hydraulic turning gear which enables the combined shaft system to be rotated manually in the event of a failure of the normal hydraulic turning gear. How to operate Manual Turning Gear : Following steps to be done

1. Remove cover 2. Then latch and attach a bar to lever. 3. Barring lever will rotate the combined turbine shaft.

After barring is completed return lever to its position and secure the lever by means of

KORBA SIMULATOR

230

latch then replace the cover. Manual barring can only be done after the shaft system has been lifted with J.O.P. OIL SUPPLY SYSTEM The oil system fulfils the following functions:

1. Lubricating and cooling the bearings. 2. Driving the hydraulic turning gear during interruptions to operation, on

startup and shut-down . 3. Jacking up the shaft at low speeds (turning gear operation, start-up and

shut-down) .

OIL SYSTEM Under normal operating conditions the main oil pump, which is situated in the bearing, pedestal and coupled directly to the turbine shaft draws oil from the main oil tank and conveys it to the pressure oil system. Two injectors aid the suction of the main oil pump. The injectors produce pressure at the suction connection to the main oil pump sufficient for all types of operation. This guarantees that the main oil pump takes over the safe supply of oil and cavitations that could occur due to greater suction heads are avoided. The amount of oil required for driving is extracted form the pressure oil circuit and adjusted by means of the throttle . The oil for the turning gear is also extracted from the pressure oil system. Oil is admitted to the nozzles by opening the shut-off valve. The pressure oil is cooled in the oil coolers and reduced to lubricating oil pressure in the throttle. The throttle is adjested on initial start-up. The amount of oil required for each bearing is adjusted on start-up by means of the oil throttles. Full-Load Auxiliary Oil Pumps During turning gear operation and start-up and run-down operation, one of the two three-phase a.c., full - load auxiliary oil pumps supplies the pressure oil system and takes over the function of the main oil pump when this as not in operation because the turbine in running too slowly. The full - load submersible auxiliary oil pumps are situated on the oil tank and draw in oil directly. Check valves behind the auxiliary pumps and in the suction line of the mian oil pump prevent oil from flowing back via pumps that are not in operation.

KORBA SIMULATOR

231

KORBA SIMULATOR

232

KORBA SIMULATOR

233

EMERGENCY OIL PUMP When main and full-load auxiliary oil pumps fail, the lubrication oil supply is maintained by a d.c. driven emergency oil pump. This pump supplies oil directly to the lubricating oil line, bypassing the oil cooler and thus preventing damage to the bearing shells. PARTIAL CONTROL OF THE AUXILIARY OIL PUMPS The full-load auxiliary oil pumps and the emergency oil pump are automatically started by the partial control as soon as the pressure switch limit has been reached. The pressure switches are set in sequence so that the pumps can be started in succession if necessary. OIL RETURN SYSTEM The lubricating oil from the bearings is returned to the main oil tank via a header. A loop in the return oil piping behind the seal oil reserve tank prevents H2 gas

reaching the main oil tank when there is a disturbance in the seal oil system. EXTRACTION OF OIL VAPOUR The main oil tank is designed to be air tight. The extractors produce a slight vacuum in the main oil tank and the bearing pedestals to draw off any oil vapour. FILTERS Oil for the thrust bearing is passed through the duplex oil filter which can be switched over and cleaned during operation. MAIN OIL TANK The main oil tank contains the oil necessary for the lubricating and cooling of the bearings and for the lifting device. It not only serves as a storage tank but also for deaerating the oil. The capacity of the tank is such that the full quantity of oil is circulated not more than 8 times per hour. This results in a retention time of approx. 7 to 8 minutes from entry into the tank to suction by pumps. This time allows sedimentation and detainment of the oil. Oil returning to the tank from the oil supply system first flows through a submerged inlet into the riser section of the tank where the first stage deaeration takes place as

KORBA SIMULATOR

234

the oil rises to the top of the tank. Oil overflows from the riser section through the oil strainer into the adjacent section of the tank where it is then drawn off on the opposite side by the suction pipe of the oil pumps. Main oil tank has the following mountings : AC auxiliary oil pump : 2 nos.

DC emergency oil pump : 1

Shaft lift oil pumps : 3 nos.

Oil injector : 1

Oil vapour extractor : 2 nos

Oil level indicator : sonar level - limit switch

FLUID LIMIT SWITCH Fluid limit switch consists of two component groups.

1. Transistorised control device 2. Individual sensor which monitors the level of the fluid in the tank

The sensor has a magneto strictively stimulated diaphragm the vibration of which is dampened when it is immersed in the fluid. In the normal state when the sensor is not immersed in the fluid, the rely in the active current circuit is attracted. When the sensor immerses in the fluid, the relay drops. The relay switches valves, control lamps or alarm devices. MAIN OIL PUMP WITH HYDRAULIC SPEED TRANSMITTER The main oil pump is situated in the front bearing pedestal and supplies the entire turbine with oil that is used for bearing lubrication, cooling the shaft journals and as primary and test oil. The main oil pump is driven direct from the turbine shaft via the coupling. This pump also conveys oil in the suction branches of the main oil pump for oil injectors, which maintains a steady suction flow to main oil pump. Hydraulic speed transmitter operates on the same principle as centrifugal pump impeller. The variation of the pressure in the primary oil circuit due to a speed variation serves as a control impulse for the Hydraulic speed governor. The hydraulic speed transmitter is supplied with control oil supplied from the control equipment rack. The suction of the pump is always flooded and hence maintains an uniform suction pressure.

KORBA SIMULATOR

235

MAIN OIL TANK

1. Oil inlet

2. Suction pipe(injector)

3. AC AOP

4. AC standby AOP

5. DC EOP

6. JOPs

7. Main section drain

8. Riser section drain

9. Oil vapour exhauster

10. Connection for oil tank level indicator

11. Connection for fluid limit switch (SONAR)

12. Inspection hole

13. Cover of entrance to riser section

14. Inspection hole

15. Oil strainer cover

16. Oil strainer

ELECTRICAL SPEED PICK UP A non magnetic disc of the electrical speed transmitter in which small magnets are inserted around the circumference gives impulses to the electrical speed pick up.When the disc rotates with the pump running, an electric current arises due to the alternating effect between the magnets and the hall generators. This current is

KORBA SIMULATOR

236

forwarded as a signal to the electrical speed pick up. AUXILIARY OIL PUMP The auxiliary oil pump is a vertical one stage rotary pump with a radial impeller and spiral casing. It is fixed to the cover of the oil tank and submerges into the oil with the pump body. It is driven by an electric motor that is bolted to the cover plate of the main oil tank. The pump shaft has a sleeve bearing in the pump casing and a grooved ball bearing in the bearing yoke. The bearings are lubricated from the pressure chamber of the pump; the sleeve bearing via a bore in the casing; the grooved ball bearing via lube line. DC BEARING OIL PUMP This is a vertical, centrifugal submerged type and serves for lubrication and cooling of the bearing during emergency conditions when one of the other pump fails. This is driven by a D.C. motor. SHAFT LIFT OIL PUMP The lift oil pump is self-priming screw-spindle pump with three spindles and internal bearings. The pump supplies the oil to lift the turbine rotor at low speeds. OIL VAPOUR EXHAUSTER The function of oil vapour exhauster is to produce a slight negative pressure in the main oil tank and in the bearing casing and thus draw off the oil vapour. The exhauster and the motor attached to it with flanges are a closed unit. The casing is constructed as a spiral with aerodynamic features and is provided with supports for the exhauster. The motor is bolted to the cover of the casing. DUPLEX OIL FILTER (FOR JOURNAL AND THRUST BEARING): It is provided to filter the oil before supply. The duplex filter consists of two filter bodies and is fitted with a changeover device, which enables the filters to be switched as desired. The filter bodies are designed according to pressure stage and the relevant codes, and the filter itself is designed to provide safety, taking into account the differential pressure which are to be seen on the service panels. The filter is provided with a differential pressure guage to give visual indication of variation in the differential pressure due to increase in filter contamination.

KORBA SIMULATOR

237

MAIN OIL PUMP

1. Threaded Ring 2. Pump Casing Upper 3. Journal Bearing 4. Oil Pipe 5. Bearing Bushing 6. Seal Ring 7. Impeller 8. Feather Key 9. Feather Key 10. Combined Journal & Thrust Bearing 11. Ring 12. Vent Pipe 13. Oil Inlet Vessel

14. Hydraulic Speed Transmitter (Governor Impeller) 15. Oil Line 16. Turbine Shaft 17. Coupling 18. Electrical Speed Transmitter 19. Permanent Magnet 20. Pump Shaft 21. Spacer Sleeve 22. Pump Casing Lower 23. Oil Tube a. Over Speed Trip. Test Oil

KORBA SIMULATOR

238

OIL COOLER Function of oil cooler is to cool the lubricating oil supplied to the bearings of turbine. Oil cooler consists of the tube nest, the inner, outer shell and water boxes. The tube nest through which the cooling water flows is surrounded by the oil space formed by the outer shell. The oil to be cooled enters the oil cooler and flows to the inner shell. This shell supports the large baffle plates which are provided with an opening in the centre. Between every two large plates there is a small intermediate plate which is held by the short tubes placed into the steel rods. The small intermediate plate is smaller in diameter than the inner shell and leaves an annular gap. This arrangement serves to achieve a cross-flow pattern forcing the oil flowing to the outlet branch to flow through the middle of the large plates, while passing round the edge of the short ones. The inner oil shell with the large plates is attached to the lower tube plate into which the finned cooling tubes are expanded. The water box with a cooling water inlet branch is bolted to the lower type plate. The tube nest is free to expand upwards in response to any thermal effect. THREE-WAY CONTROL VALVE The three-way control valve is electrically driven and has the function of regulating the lubricating oil temperature at 450C. Possible oil flow paths for regulating the oil temperature :

1. All lubricating oil flows through oil cooler

2. Lubricating oil flows through oil cooler and by-pass piping

3. All lubricating oil flows through the by-pass piping.

HP CONTROL FLUID SYSTEM HP CONTROL FLUID PUMP The extraction or dual pressure pump is a vertical rotary pump in multiple stages. It is attached to the cover of the fluid tank and submerges in the control fluid. Driven by a electrical motor located on the cover plate of the tank. After one stage oil is delivered to LP control fluid circuit and the HP control fluid is taken after 4 stages of the pump. The pump shaft is guided by a sleeve bearing in the suction casing and by ball bearings in the bearings in the bearing support. The bearings and the bevel gear coupling are lubricated from the first stage pressure chamber, the sleeve bearing via a passage in the casing, the bearings and the bevel gear coupling via the lubricating pipe.

KORBA SIMULATOR

239

CONTROL FLUID TANK FUNCTION The control fluid tank contains the fire resistant control fluid necessary for the governing system of the turboset. A part from its function of storing, it also deaerates the control fluid. The tank is designed so that the entire contents can be circulates a maximum of 8 times per hour. This results in the control fluid remaining approximately 7 to 8 minutes in the tank in which time any air in the fluid can be separated and any ageing materials deposited. FLOW OF CONTROL FLUID IN TANK The control fluid returning from the governing system enters the tank through the control fluid inlet (8) and flows into the riser chamber of the tank where the first deaeration takes place. The control fluid then drops through the strainer (5) into the adjacent chamber and flows to the control fluid pumps which conduct the fluid to the governing system. CONTROL FLUID PUMPS The control fluid pumps (1,2) are situated on the control fluid tank and immerse with the pump bodies into the control fluid in the tank [1]. They draw from the deepest point in order to conduct control fluid that is as free of air as possible. The driving elements of the pumps are fixed to plates on the cover of the tank. CONTROL FLUID STRAINER The control fluid strainer (5) is a basket strainer installed in the tank. It is 0.28 mm wire mesh and can be exchanged by opening the hatch (12). REGENERATING CIRCUIT The control fluid is cleaned and regenerated in a separate circuit. A constant amount of fluid is conducted through a fuller’s earth filter and a mechanical filter by a circulating pump and returned to the tank. CONTROL FLUID INDICATOR The control fluid tank is provided with a local fluid level indicator [2] and level switches [3] with which the maximum and minimum levels of the control fluid can be transmitted. A storage space is provided between the operating level of the control fluid, which corresponds to the nominal contents of the tank, and the tank cover.

KORBA SIMULATOR

240

FRF TANK

1. Electrically Driven Control Fluid Pump

2. Electrically Driven Stand By Pump

3. Ventilation Of Drain Piping

4. Fluid Level Monitor

5. Strainer Body

6. Main Chamber Discharge

7. Riser Chamber Discharge

8. Control Fluid Inlet

9. Observation Hole

10. Man Hole Chamber

11. Water Indicator

12. Strainer Hatch

13. Heater

14. Fluid Limit Switch

15. Water Indicator

16. Observation Hole

17. Ventilation Of Tank

This can accommodate the fluid in the entire control fluid system when the turbine is

KORBA SIMULATOR

241

shut down. The floor of the tank is sloping with discharge facilities (6,7) at the deepest points. PROTECTION DEVICE Special electrodes (11,15) signal immediately any undesired penetration of water. REGENERATING PLANT Of the various types of the fire resistant fluid, the only ones suitable for use with KWU turbines are phosphoric esters of the group HS-D which have a low water and chlorine content. Their chemical composition and structure necessitate certain measures and alterations compared with an oil system. Fire resistant fluid systems for KWU turbines are provided with a bypass regenerating plant. The design of this plant is made to the specifications of the fluid manufacturer. Any acids and ageing products are removed during operation by the continuous filtering through Fuller’s earth and mechanical filters. The mode of operation of this natural earth treatment is based on a ion-exchange reaction. In addition to the precautions against acidifying of the fluid, continuous care is taken that any solid particles are separated by the fine filter so that they can not speed up the reaction. The fine filter of this plant retains particles of Fuller’s earth as well as providing the essential cleanliness of the whole system and increasing the life of the filters. The Fuller’s earth needed for regenerating the fire resistant fluid must be dry (at 150OC the amount of expellable water must only be 1 % of the weight). The US strainer number 30/60 mesh is the granular size to be used (or this must correspond to the details from the fluid manufacturer). The dust proportion of the granulate must not be used. The amount of earth must not be too little and must be stamped or shaken to avoid the formation of gaps and channels which would reduce the effectiveness of the Fuller’s earth. The efficiency of the regenerating plant is to be controlled by an exact record of the neutralisation and the degree of purity. CONSTRUCTION OF REGENERATING PLANT The filter group consists of 2 Fuller’s earth filters (6) and a mechanical filter (7). The

KORBA SIMULATOR

242

cleaning and deacidifying takes place in a separate circuit. A pump (4) conducts a constant amount of fluid through the filter group into the tank (1). When the filter is contaminated there is an increase in the fluid pressure. A spring safety valve (3) is installed to protect the system against an excessively high increase in pressure.

ARRANGEMENT OF REGENERATION PLANT

1. Control Fluid Tank

2. Control Fluid Pump 32/8 Bar

3. Safety Valve

4. Circulating Pump

5. Shut-Off Valve

6. Fuller’s Earth Filter

7. Strainer

a. Control Fluid Approx. 8 Bar

A1. Control Fluid Approx. 32 Bar

C. Return Flow

C1. Riser Room Drainage

C2. Main Room Drainage

FULLER’S EARTH FILTER The Fuller’s earth filter contains three sections with a special granulate which binds the acid present. Two filters work in parallel and can not be switched over.

KORBA SIMULATOR

243

MECHANICAL FINE FILTER Following the Fuller’s earth filters is a fine filter with textile inserts of finest mesh. These inserts retain the finest particles of dirt, both metallic and non-metallic impurities. In this way the fine filter also serves the safety of the control fluid system by trapping any particles of granulate that may be circulating. The fine filter also separates water particles and other ageing materials which would make it necessary to renew the control fluid too soon. COMMISSIONING AND MAINTENANCE OF THE FILTER PLANTS

• The filter are to be continuously deaerated by slightly opening the vent valves.

• Observe pressure increase on pressure gauge.

• The acid content must be checked by examining the fluid in the laboratory at certain intervals.

• If there is a constant increase in the acid value, the granulate is exhausted and

must be replaced earlier than originally intended.

• The increasing contamination of the filter can be measured by the constant

increase in pressure. The differential pressure indicators installed in the system show optically the degree of contamination. When the white-red segments are only showing red, the filter material must be changed.

EXCHANGING THE EARTH FILLING

• Switch off pump

• Open filter drain (6). Drain filter

• Loosen cover bolts

• Lift cover - do not damage seal

• Loosen drain pipe screws and lift basket cover

• Extract filter basket (7) carefully and centrally

• Clean inside of filter casing

KORBA SIMULATOR

244

• Clean filter basket (7)

• After cleaning care must be taken that the parts are completely dry.

• Fill filter basket and insert. The earth must be carefully pressed down-without

force- so that the sections are filled compactly in order to prevent later settling of the earth and the possible formation of channels.

• Insert clean basket cover and tighten well with screw so that no Fuller’s earth

can escape.

• Close drain (6).

• Replace cover carefully and tighten uniformly by means of screws (pay attention to seal)

• Switch on pump (4)

KORBA SIMULATOR

245

TURBINE GOVERNING SYSTEM

KORBA SIMULATOR

246

KORBA SIMULATOR

247

TURBINE GOVERNING SYSTEM HYDRAULIC SPEED GOVERNOR FUNCTION The function of the hydraulic speed governor is to operate the control valves to give the appropriate turbine steam through put for the particular load condition. The arrangement and functioning of the governor within the overall governing system is described in the section on governing. CONSTRUCTION The principal components of the speed governor are the bellows (8), the link (11), the speed setting spring (13), the sleeve (5) and the follow-up piston (4). The primary oil supply from the hydraulic speed transmitter is available at connection a1. A fire resistant fluid is used as the hydraulic fluid in the governing system. An additional bellow (9) prevent primary oil getting into the control fluid circuit should there be a leakage in the governor bellows (8). In this case, the leakage oil can be drained off via connection c1. Should a leak in the bellow (9) occur, the control fluid that has leaked in will also be drained off via connection c1. The primary oil pressure (connection a1) is dependent on the speed and determines the position of the link (11) via the bellows (8) and the pushrod (10). The speed setting spring (13) opposes the primary oil pressure. Its pre-compression can be varied either by hand or remotely by the motor (16). The sleeve (5) which can slide on the bottom end of the follow-up piston is held against the auxiliary secondary fluid pressure (connection b) by the tension spring (3). The follow-up piston and the sleeve have ports which at normal overlap allow sufficient fluid to escape to produce equilibrium between the auxiliary secondary fluid pressure and the force of the tension spring (3). Each steady-state position of the link (11) and hence of the sleeve (5) corresponds to a specific force from the tension spring and hence to a specific secondary fluid pressure which in turn determines the position of the control valves. MODE OF OPERATION If the primary oil pressure falls (as a result of increasing load and the resulting drop in speed), the link (11) and the sleeve (5) sliding on the follow-up piston (4) are moved downwards by the speed setting spring (13) so that the overlap of the ports in the sleeve and the follow-up piston is reduced. This causes the pressure in the secondary fluid circuit to rise and the follow-up piston follows the movement of the sleeve against the increasing force of the tension spring (3) unit normal overlap of the ports and equilibrium are restored. The lift of the control valves is increased in this manner by the increased secondary fluid pressure. Conversely, a rise in primary oil pressure causes the lift of the control valves to be reduced.

KORBA SIMULATOR

248

HYDRAULIC SPEED GOVERNOR WITH

STARTING AND LOAD LIMIT DEVICE

When the pre-compression of the speed setting spring (13) is varied with the reference

KORBA SIMULATOR

249

speed setter it changes the relationship between the primary oil pressure and the secondary fluid pressure and hence the relationship between speed and power output. The zero position of speeder gear corresponds to 2800 rpm i.e. hydraulic governor comes into action after 2800 rpm. Lever (12) allows the link (11) to be depressed by hand to give a lift signal to the governor, e.g. to provide a second means of overspeeding the machine for testing the overspeed trips in addition to the overspeed trip tester.

AUXILIARY FOLLOW UP PISTONS

Auxiliary follow up pistons are two in

number and are connected parallel. The

trip oil is supplied through orifices to the

aux. follow up valves. The sleeves of the

valves are attached to the speeder gear

bellow link. The sleeve position

determines the drain of trip oil through

the aux. follow up pistons. Accordingly

the trip oil pressure, upstream of these

valves changes. Upstream of circuit is

termed as Aux secondary oil circuit.

Hence Aux. follow up pistons are said to

control Aux secondary oil pressure.

STARTING AND LOADING LIMITING DEVICE Before start-up, the pilot valve (21) is brought to its bottom limit position either by hand or remotely by the motor (20). This causes the bellows to be compressed via the lever (6) and the pin (7) until the governor assumes the position “control valves closed”. With the valve (21) in the bottom limit position control fluid from connection a can flow simultaneously to the auxiliary starting fluid circuit (connection u1) and as starting fluid via connection u to the stop valve to prepare these for opening. When the valve (21) is moved back the auxiliary starting fluid circuit is depressurized and subsequently the starting fluid connection u is opened to the return c. This opens the stop valves. Further upward movement of the valve (21) causes the pin (7) to release the bellows as with falling primary oil pressure and the control valves are opened. The

KORBA SIMULATOR

250

release of the bellows can be limited by the pin (7) so that the control valves do not open any further despite a further reduction in primary oil pressure. LOAD LIMITING. The start-up and load limit device acts mechanically on the bellow of the hydraulic speed governor, allowing it to function as load limiting device as well, i.e., lift of the main steam control valve,reheat steam control valves to an adjustable value. This can be done either manually or by motor of starting and load limit device. MAIN TRIP VALVE. The function of the main trip valve is to open the trip oil circuit in the event of abnormal conditions, thereby closing the main and reheat stop and control valves and thus shutting off the admission of steam to the turbine.

MAIN TRIP VALVE 1: Limit Switch

2: Spring

3: Piston

4: Body

I: Trip Medium

II: Aux. Trip Medium

III: Drain Medium

IV: Control Medium

V: Aux. Start-Up Medium

KORBA SIMULATOR

251

RESETTING MAIN TRIP VALVE AFTER A TRIP OUT. When the starting device is lowered to minimum position (0 %), control oil flows via. the pilot of starting device as Aux. Start up oil, which acts at the bottom of the main trip valve for lifting the pilot. As the pilot of the main trip valve is raised to normal (resetting) position, control oil flowing via main trip valve, generates trip oil and aux. trip oil through main trip valve. The aux. Start-up oil also act at the piston (A & B) of the over speed trip device and thrust brg. trip device for resetting. The aux. trip oil is blocked in the reset position. REMOTE TRIP SOLENOID Remote trip solenoids are two in numbers and are connected in Auxiliary trip fluid circuit in parallel. During normal operation the remote trip solenoids blocks the passage of Auxiliary trip fluid medium to drain. When any electrical trip condition of the turbine comes then the remote trip solenoids gets energised and connects the Auxiliary trip fluid medium to drain.

REMOTE TRIP SOLENOID

1. Compression Spring

2. Magnet System

3. Body

4. Vent Hole

I: Auxiliary Trip Fluid

II: Drain Medium

KORBA SIMULATOR

252

OPENING OF STOP VALVES STEP-I. When

• Starting device is lowered to 0 % position; start up oil (via. Starting Device) generated from control oil presses the test valve pilot down.

Next

• Trip oil flows to the top of ESV, IV servomotor;

• Servo piston is pressed down to rest over piston plate against compressible spring

• Oil in between piston and piston plate as well as below piston is connected to

drain via test valve.

STEP -II. When

• Starting device is raised control oil supply line gets blocked and start up oil line is connected to drain;

• Test valve pilot moves upwards, trip oil flows to the bottom of servo-motor

piston; • Piston and piston plate set slowly moves up while the trip oil above the piston

slowly drains via test valve;

• Stop valve slowly opens to 100 % position. At 42 % position of Starting device HP Stop valve opens and at 56 % position of Starting device IP Stop valve opens.

CONTROL SYSTEM. Starting procedure. The turbine is started and brought upto synchronisation speed by means of main steam control valves and reheat steam control valves. The speeder gear is set at the “minimum speed” position, if it is intended to operate the turbine with the hydraulic speed governor. In this case the reference speed of the electro-hydraulic controller is at the “maximum speed” position. If on the other hand, start up is to be effected with electro-hydraulic controller, the speeder gear is set at “maximum speed” and the reference speed of the electro-hydraulic controller at “minimum speed”. The main stop valves and control valves are still closed, since the trip fluid circuit is not yet pressurised. First, the tension of the spring in the follow up piston is released by a lever system by means of hand wheel or motor of the start-up and load limit

KORBA SIMULATOR

253

device (By turning hand wheel clockwise or by operating motor in “closed” direction), so that no secondary fluid pressure build up in the follow up piston. The converter for speed governing along with follow up piston is now in the “control valves closed” position. Thus there is no secondary fluid pressure build up, when the main trip valves are latched in. By further turning of the handwheel, the pilot of the starting and load limit device is moved further downward, and the pressure fluid is first admitted into the start up fluid circuit and then into the auxiliary start up fluid circuit. The start up fluid flows into the space above test valve and forces them downward against the spring load. The auxiliary start up fluid lifts the positions of the main trip valves thereby allowing the trip fluid to flow to test valves of the main steam stop valves and reheat steam stop valves. The trip fluid can now flow to space above the pistons of the main steam stop valves and reheat steam stop valves, pressing them onto the lower piston disc. Operation of the start up and load limit device is continued until it attains lowest position. Subsequently, by turning back the handwheel or by operating the motor of the start-up and load limit device in the “Open” erection. The pressure fluid is first discharged from auxiliary start up fluid circuit and then from the start up fluid circuit. The pistons of the test valves move upward by springs and as a result trip fluid builds up in the space below piston and is slowly discharged from the space above piston. The resulting differential pressure permits both pistons to move together to the upper limit position thus causing main steam stop valves and reheat steam valves to open. fluid pressure now holds the main trip valves below the stop of differential piston. When the “open” position of main steam stop valves is reached, further turning of handwheel or operation of the motor in the “open” direction after a certain amount of backlash, will enable the speed governor diaphragm to move downwards, causing increase in the auxiliary secondary fluid pressure. This would cause the opening of control valves through various systems and enable the turbine to reach upto 85 to 90 % of rated speed. At this time hydraulic speed governor will cut in and maintain this speed. The start up and load limit device is then set at “Fully open” position. An electric speed transmitter continuously measures and indicates the turbine speed. Further increase of speed, paralleling and loading of the turbine are done by speed changer. Turning of the handwheel of the speeder gear or operation of its motor causes the speeder spring to be compressed further, which in turn results in an increase of speed. When turbine is in grid, the system determines the turbine speed, and operation of the speed changer leads to corresponding load change on the turbine.

KORBA SIMULATOR

254

HYDRAULIC SPEED CONTROL. On the shaft of main oil pump, the “ Hydraulic speed transmitter” has been provided, which provides primary oil pressure. The change in absolute pressure can be taken as proportional to small changes in speed (with in limits of steady state characteristics). This primary oil pressure acts on diaphragm of “Hydraulic speed governor” against the force of speed setting spring, which is compressed by speed changer. Travel of diaphragm may be limited by starting and load limiting device. The movement of diaphragm is transmitted by link mechanism to “Auxiliary follow-up piston”. The position of “Auxiliary follow up pistons” is held in balance by a spring against fluid pressure. The fluid is essentially trip fluid fed from trip circuit and drained from a port formed between the piston and the sleeve. Depending upon port opening fluid pressure gets stabilised corresponding to initial displacement of the piston and initial spring tension. The auxiliary secondary fluid pressure provides a signal to “Hydraulic Amplifier” through its pilot.

1. Amplifier piston 2. Follow-up piston 3. Sleeve 4. Shaft 5. Lever 6. Feedback lever 7. Pilot valve 8. Compression spring 9. Adjusting screw a : Control oil b : Secondary oil b1 : Aux. Sec oil c : Return oil

HYDRAULIC AMPLIFIER

KORBA SIMULATOR

255

HYDRAULIC AMPLIFIER It 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 the amplifier piston is connected to the drain. The movement of the amplifier is 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.sec oil pressure. FOLLOW UP PISTONS: The trip oil is supplied to the follow up pistons through orifices and flows in the secondary oil circuits to control valves. The movements of Hydraulic amplifier through lever assembly mentioned above actuates the sleeves of the follow up pistons (6nos.) via. Link . A secondary fluid pressure corresponding to the position of the sleeve and related spring tensions is built up in the “Follow-up Pistons” of the “Hydraulic Amplifier” in a similar fashion to Auxiliary follow-up pistons.Three follow up pistons are provided for HP Control valves and three follow up pistons for IP Control valves. Any change in the position of link results in proportional change of secondary fluid pressure in the follow-up pistons of Hydraulic Amplifier. The secondary fluid circuit is also fed from trip fluid circuit through reducing valves. The varying HP/IP Secondary fluid pressure in the follow-up pistons of Hydraulic Amplifier operates the control valves. A feed back system has been foreseen for quickly stabilising the position of the pilot valve and the piston of Hydraulic Amplifier. With a view to avoiding sticking of pilot spool and increase sensitivity pilot spool is kept rotating due to reaction of fluid leakage through tangential holes. ELECTRO-HYDRAULIC CONVERTER The electro-hydraulic converter is used as the connecting link between electrical and hydraulic parts of the turbine governing system. It converts electrical signals from electrical governor into hydraulic signals and amplifies them for actuation of control elements. Electro-hydraulic converter consists mainly of permanent magnet plunger coil system with sleeve, the control pilot valve, the power piston the follow piston and an electrical feed back system.The control signal from electrical governor actuates the sleeve through plunger coil system. The sleeve slides on the upper part of pilot valve and decides the position of pilot valve as in the case of follow-up pistons. Pilot valve and

KORBA SIMULATOR

256

sleeve have ports through which trip fluid from inlet drains depending upon the overlap. In steady condition the pilot valve is in the middle position. Trip fluid pressure and spring force are in balance. The pilot valve rotates due to the flow of control fluid through tangential holes in one of its bobbins. It increases the sensitivity of pilot valve and ensures its free movement.When pilot valve moves from its middle position, control fluid flows at the top of bottom of power piston and the opposite side is connected to drain. The power piston moves and actuates sleeves through the lever which causes movement of corresponding follow-up pistons. Secondary fluid circuit is supplied by trip fluid circuit through throttles. The secondary fluid flows to control valve servomotors for control purpose. The secondary fluid pressure depends upon the tension of spring which balances the secondary fluid pressure on follow-up piston. Follow-up piston and sleeve have slots through which secondary fluid flows depending upon the overlap. The port area changes due to movement of sleeve and therefore secondary fluid pressure in follow-up piston also changes such that it follows the movement of sleeve. The balance is again achieved in the tension of spring and new secondary fluid pressure. Each position of power piston corresponds to a definite position of sleeve and that of follow-up piston. The position of follow-up piston determines the secondary pressure. The initial tension of follow up piston springs can be adjusted with the help of adjusting screw. CONTROL PROCESS WITH ELECTRICAL GOVERNOR. When electrical governor gives impulse for opening of control valves, the sleeve of plunger coil system moves up and drain area through sleeve reduces. The pressure under the pilot valve increases and results in upward movement of pilot valve. Thus the control fluid supply is available at the bottom of power piston while its top is connected to drain. The upward movement of piston moves the sleeves through levers downwards and reduces the drain area between sleeves and follow-up pistons such that the pressure in the follow piston and so in secondary fluid circuit increases. The feedback of movement of power piston is provided through differential transformers to control pilot valve. The sleeve moves back to such an extent that in the new position of power piston, the pilot valve takes its middle position and balance is again achieved between pressure under pilot valve and spring force. When electrical signal is given for closing the control valves the process occurs in the reverse order. SELECTION OF GOVERNORS: Both Hydraulic and Electro Hydraulic converter follow up pistons are generating HP/IP secondary oils.They constitute a minimum 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.

KORBA SIMULATOR

257

1. Differential Transformer

2. Amplifier Casing

3. Amplifier Piston

4. Piston Rod

5. Valve Bushing

6. Pilot Valve

7. Grooved Ball Bearing

8. Spring Disc

9. Compression Spring

10. Sleeve

11. Casing Support

12. Moving Coil System

a Control Fluid

X Trip Fluid

ELECTRO HYDRAULIC CONVERTER

CHANGEOVER FROM HYDRAULIC TO ELECTRO HYDRAULIC CONTROL. As earlier pointed out, changeover from one control system to the other one is possible during normal operation of the turbine since the two systems are brought into parallel connection after associated follow up pistons which represent a minimum value selection, meaning that the system with the lower reference value is always the controlling one.If turbine is to be run under hydraulic control the reference speed of the electrical controller is at “Max. Speed” which prevents the Electro-hydrualic system from coming in action.

KORBA SIMULATOR

258

When bringing in the electro-hydraulic control system the reference speed of the electrical controller should be reduced slowly until the secondary fluid pressure drops

FOLLOW-UP PISTON VALVES WITH HYDRAULIC AND ELECTRO-HYDRAULIC CONVERTER AND TRIMMING DEVICE

slightly. When this occurs, the electro-hydraulic converter has taken over. The speed changer of the hydraulic speed governor is then set at maximum speed. The electro-hydraulic converter is now fully effective and can operate over the entire output range. The hydraulic speed governor also acts as a speed limiter in the event of electrical controller developing a fault. In this case, operation of turbine may immediately be continued by means of hydraulic speed governor. CHANGEOVER FROM ELECTRO HYDRAULIC TO HYDRAULIC CONTROL. This changeover is done in the reverse sequence mentioned above. First, speed changer is actuated in the “decrease” direction until secondary fluid pressure drops slightly. This is an indication that hydraulic speed governor has taken over the

KORBA SIMULATOR

259

control. Then reference speed limit of electric controller is set at “maximum”. Now the hydraulic speed governor is fully effective, and can operate over the entire load range. The secondary fluid pressure is transmitted to the actuators of the HP & IP control valves and controls their opening. SERVOMOTOR FOR MAIN STOP VALVES AND REHEAT STOP VALVES The operative part of the servomotor consists of a two part piston the lower disc-shaped part of which is connected via piston rod to the valve stem. The other part of the piston is bell-shaped and moves within the housing which is in the form of a cylinder. Two spiral springs are placed between the two valves of the piston at the lower end a spring plate is interposed between the springs and the piston disc. When trip fluid is admitted to the space above the bell-shaped part of the piston, it moves this half of the piston downwards, compressing the springs, until it seats against the piston disc. After the main stop valves have been opened, the turbine is started by the control valves. Before the main stop valves can be opened, however, they must be “pressurised”, i.e. prepared for opening, by admitting trip fluid from the trip fluid circuit to the space above the piston to press it down against the piston disc after overcoming the resistance of the springs. The edge of the bell-shaped half of the piston is designed to produce an fluid-tight seal with the piston disc. TEST VALVE: Each of the HP/IP stop valve servomotors receive trip oil through their associated test valves.The test valves have got port openings for trip oil as wall as start-up oil.The test valves facilitate supply of trip oil pressure beneath the servomotor disc(stop valve open condition) under normal conditions.For the purpose of resetting stop valves after a tripping,start up 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 pressure beneath the disc gets connected to drain.When start up oil pressure is reduced the test valves moves up draining trip oil above the servomotor piston and building the trip oil pressure below the disc, thus opening the stop valves.A hand wheel is also provided for manual operation of test valves. To open the valve, fluid from the trip circuit is admitted to the space below the piston disc and, simultaneously, the space above the bell-shaped half of the piston is opened to drain. This causes both halves of the piston to move together in the direction which opens the valve. In order to reduce fluid leakage past the bell-shaped part of the piston when the valve is open, a back seat is provided in the housing against which the collar of the piston can seat. When the valve is tripped, the pressure in the trip fluid circuit, and hence in the space below the piston disc, falls with the result that the springs separate the two halves of the piston and the piston disc connected to the valve stem move to close the valve. Just before the valve disc seats, the piston disc enters a part of the cylinder where the

KORBA SIMULATOR

260

diametrical clearance is reduced.This arrangement restricts the flow of fluid past the piston disc and so produces a braking action which causes the valve disc to seat gently. All fluid connections are routed through a test valve. All operation can be controlled by means of the test valve and the starting and main trip valve.

x Trip fluid to space below the

piston

x1 Trip fluid to space above the

piston

c return

STOP VALVE SERVOMOTOR MAIN AND REHEAT CONTROL VALVE SERVOMOTOR The flow of steam to the turbine is regulated by varying the limit of control valves by means of their servomotors. Control valves have been provided for HP turbine and IP turbine. All the control valves are operated by their individual servomotors, which are actuated by the high-pressure control fluid supply at 32-bar approx. The control valve is moved by the piston, which is loaded by disc springs on one side and by hydraulic fluid pressure on the other side. The position of the valve is determined by the secondary fluid pressure, which is determined by the governor. Since large operating forces are required the servomotor is of high-pressure type (approx. 32bar and has a pilot control system. The secondary fluid supply at connection ‘b’ controls the position of auxiliary pilot valve, which directs control fluid from connection ‘a’ to the appropriate side of the pilot piston.

KORBA SIMULATOR

261

The pilot piston operates the main pilot valve through lever so that when the control valve is being opened, the control fluid at connection ‘a’ is directed to the underside of the piston. When the control valve is being closed, control fluid from the under-side of the piston drains through the main pilot valve. PILOT CONTROL SYSTEM When the turbine is running at steady load, control valve will also be steady at a particular value of valve lift corresponding to the load and the auxiliary pilot valve will be in the centre position as shown in the figure. In this position, the force exerted by the spring is balanced by the secondary fluid pressure acting on the auxiliary pilot valve. When the load increases, the secondary fluid pressure gets increased by the action of the governor, thus opening the control valves. This action is reversed, when the load decreases. In either case, the auxiliary pilot valve is deflected from its centre position. This allows control fluid at connection ’a’ , to flow to one side of the pilot piston, while the other side of the piston is opened to drain. The movement of the pilot piston returns the auxiliary pilot valve to its centre position by means of feed back linkage, thus giving proportionately between secondary fluid pressure and pilot piston travel. The degree of proportionality of the pilot control system can be adjusted by altering the pivot position of the feed back linkage. MAIN CONTROL SYSTEM The movement of the pilot piston deflects the main pilot valve from its centre position by means of lever so that either the control fluid from connection ‘a’ is allowed to enter the under side of piston, thus opening the control valve or the underside of piston is opened to drain thus closing the control valve by the force of the disc springs. Just before the control valve disc actually comes into contact with the valve seat piston enters a recess provided in the servomotor casing and throttles the flow of fluid draining from the underside of the piston. This slows down the motion of control valve at the time of closing and thus the closure of the valve takes place smoothly. The spring preloads the linkage and prevents any slackness or erratic movement at the pivots. Feed back cam mounted at the end of the piston rod brings the main pilot valve to its centre position by means of a lever system. The slope of the feed back cam is in two stages, which gives two degrees of proportionality, thus producing good linearity of the steam flow characteristics.

KORBA SIMULATOR

262

KORBA SIMULATOR

263

HYDRAULIC SERVOMOTOR FOR MAIN CONTROL VALVE

KORBA SIMULATOR

264

ELECTRO - HYDRAULIC GOVERNING SYSTEM. The turbine is equipped with Electro-hydrualic Governing system to facilitate the operation of the Turbo-set in an inter-connected grid system measuring and processing of signals offer the advantages such as felxibility, dyanmic stability and simple representation of complicated functional relationship. The processed electrical signal is introduced at a suitable point in the hydraulic circuit through Electro-hydraulic converter. The hydraulic controls provide the advantage of continuous control of large positioning forces for control valves. The integration of electrical and hydraulic system offers the following advantages:

• Exact load frequency droop with high sensitivity

• Reliable operation in case of isolated power grids

• Dependable control during load rejection

• Low transient and low steady-steady-state speed deviations under all operational conditions.

• Excellent operational reliability and dependability

• Safe operation of the Turbo-set in conjunction with the Turbine stress evaluator

(TSE) ELECTRO-HYDRAULIC CONTROLLER The parameters to be controlled i.e. speed, load and turbine throttle pressure are measured and converted into electrical signals by means of suitable transducers. These signals are then fed to various control loops where they are processsed as per requirement of the operator (reference value signal set points) and the turbine conditions (monitoring signals). The control is converted into hudraulic signals for actuating the hydraulic operated controller basically consists of the subsidiary controllers for main steam control valve lift (valve position controller), speed, load and boiler pressure are supper imposed to main steam control valve position loop. SPEED CONTROLLER The speed controller is used for starting the turbine upto synchronisation and block loading. It can also operate over the full load range during emergency such as generator tripping from full load to house load operation or during rapid load throw-offs and / or severe frequency fluctuation. The speed controller is always kept in readiness for operation even when it is not directly controlling the turbine by tracking the signal generated by the other controller in service.

KORBA SIMULATOR

265

The speed reference value can be chaged from- (a) desk, (b) control cubicle, (c) ATRS, (d) Synchroniser equipments. SPEED CONTROL LOOP Speed reference value is set by means of potentionmeter operated from remote (UCB control panel) or manually (control cabinet) normally in the range of 0-3000 rpm (0-9V). Above a speed of 47 Hz (2830 rpm) a reducing gear box lowers the speed of potentiometer to 1/4th, to facilitate exact adjustments, of speed. The speed reference value is indicated on two instruments, one with a range of 0-3300 rpm and other with range of 2700-3300 rpm (for finer adjustment). The control device for the speed reference value generates the time dependent speed reference value NRTD/NRLIM which influences the speed controller. As shown in the fig., NR is fed to a highgain DC amplifier. The subsequent intergrator responds to a very small imbalance of the imputs of DC amplifier. Output of the integrator is NRTD/NRLIM which chages like a ramp. The speed reference value is tracked depending on two operating conditions:-

a) As soon as the Load Controller or pressure controller has taken over within a frequency band of 49 Hz-51Hz (adjustable), Speed controller tracks the actual grid speed with an offset of 15 rpm so as to take care of house load operation in case of opening of generator breaker.

b) As soon as emergency stop (turbine trip) acts, the speed reference value is

tracked with an offset (Nact-120) r.p.m. The emergency is memorised in a biased memory. The resetting is effected manually or by means of “increase” command for speed reference value. The gradient of signal NRTD however is modified according to available TSE margins. The TSE influence is introduced between DC amplifier and the integrator to limit the input voltage to the integrator to the level up allows a rise of turbine speed at highest possible rate consistant with the conservative permissible operation of turbine on account of the level of thermal stresses. The freezing of the speed reference value (when Nact > 2850 rpm) can occur for the following two cases:-

a) If TSE becomes faulty the reference value is blocked and integrator stops. The integrator can again be enabled after the TSE influence is switched off and master set point release push button is reset. Since the stress evaluator monitors the dynamic margin on fault detection, the fault is stored in the turbine governor. The memory is reset by command; “Stress evaluator limitation off”.

KORBA SIMULATOR

266

SPE

ED

CO

NT

RO

LLE

R

KORBA SIMULATOR

267

b) On tranistion from turning speed to control by electric speed controller, a transition period occurs as a result of undermodulation of the speed by speed controller. The reference value is blocked at Nref - Nact > 50 rpm in order to prevent occurance of excessive control deviation (N).

If the speed set point control is in action, the test function of the stress evaluator is suppressed. The NRTD signal is transmitted to the final speed controller where it is compared with actual measured speed signals and thus generate final controller output of speed controller. NO-LOAD SPEED CORRECTION As a result of proportional control behaviour of the speed controller, a control error exists between the actual value & the reference value. A feed forward signal which is influenced as a function of boiler pressure is provided to achieve identical speed at synchronising point. DROOP OF SPEED CONTROLLER 5% i.e. 2.5 Hz above and below 50 Hz. So for a changing of + 150 rpm of reference or speed, the speed controller ouput will vary from 0 to + 10 volts. STOPPING OF SPEED REFERENCE VALUE CONTROL If stress evaluator fault occurs and the generator circuit breaker is “open” and NR<NRTDthe speed reference value control is stopped. When the generator circuit breaker is “closed” stoppage of the speed reference value control is inhibited. The speed reference value control is also stopped in an analogous manner to that described above when the unit is operating in pressure control mode. SPEED MEASUREMENT AND PROCESSING Rotational speed of turbine is measired digitally with the help of 3 nos. of electromagnetic digital pick-ups, (Hall Probes) which are mounted on turbine shaft inside front bearing pedestal. These comprises axial, stationary sondes utilising hall effect and a non-magnets disc rotating with turbine shaft carrying small permanent magnets with alternative polarity in equally spaced hole arranged in concentric circles. an air gap separates the magnets and sondes. The output of probes is transmitted to an impulse conversion device which contains three signals processing channels and furnishes three times, three electrically segregated perfect square wave voltages + 10%. Each one of the square wave voltages of impulse conversion device per channel is subsequently processed in the speed

KORBA SIMULATOR

268

controller module, to obtain actual value of rotational speed. Actual measured speed values are continuously monitored for failures. In case of failure of anyone channel, the control circuit continues to operate without interruption case of speed measurement control failure of EHC, the tracking controller is switched off automaticlly. The reliable pulse frequency thus obtained is fed into three speed mesuring devices. The first device consists of F/V converter and operates in the range of 0-60Hz. Second device processes the pulse frequency in two ranges 0-6 Hz and 0-60 Hz for the equipment generating analog signal as “Acquisition of actual speed channel no. 1”. Third speed measuring device comprises a quartz frequency standard, a pulse amplifier, a circuit for evaluation of difference between the frequency standard and measured grequency, a F/V converter, a measuring adapter and limt signalling circuit. the device operates between 45 Hz and 55 Hz and is required for formation of droop. The speed controller exhibits PDP response i.e. speed controller is a PD Controller with sloping characteristics. The output of speed controller remains proportional to control deviation during stationary operations. This arrangement results in better load sharing by several turbo sets connected to the same grid as compared with purely mechanical and hydraulic governor. DN/DT MONITORING During rolling of the turbine, if, between the speeds 600 rpm to 2829 rpm the rate of speed rise is very low i.e. less than 108 rpm/min. then DN/DT monitoring operates giving appropriate alarms in the UCB. It blocks any further rise in speed and brings back the speed reference to 600 rpm. this is incorporated to avoid low acceleration rates when the turbine speed lies in the critical speed rage 2850 rpm). LOAD CONTROLLER The load controller is used for controlling turbine output during load operation. for selecting the load controller a push button module LOAD CONTROLLER, ON/OFF is provided. The load controller must be switched ON if it has to come into action. for final load control, several factors like load Reference (PR), Load Limiter Set point (PRmax.), TSE influence, Frequency Influence and Pressure Influence are considered and a final reference PR is developed. an indication “LOAD CONTROLLER ACT” is provided on the turbine control panel when this lamp glows, which show that the load controller is in service. The power reference value can be changed by:-

a) Manually from desk

b) From the CMC control desk

KORBA SIMULATOR

269

c) From the control cubicle

The loading and unloading gradients are influenced by

a) Turbine stress evaluator or

b) Rate of power change of reference value from desk

The load controller is designed for two modes of operation

a) Power operation in conjunction with power system with PI control characteristics.

b) Isolated grid operation with P control characteristic based on the frequency of

the isolated power system. Isolated power system operation is based on power control deviation.

LOAD CONTROL LOOP

Load reference value is set from reference setter module on the turbine panel, by means of motorised potentiometer and the output of reference setter PR is displayed

by the UCB desk indicator. The device for the load reference value contains proportional (P) channel parallel to I-channel. On account of this addition the response of the device is proportional to small changes of load reference value and for large changes it is proportional integral. Control device for load reference value generates time dependent load reference value (PRTD/PRLIM) which influences the

load controller. This signal rises during start-up at a rate (MW/min) selected through load gradient setter until the final load reference value PR has been reached. The rate of change of PRLIM is also subjected to the additional limits because of influence of TSE margins if the same is in switched-on condition. The indications of PRLIM and gradient setter are provided on the UCB panel. If the rate of rise of PRTD is limited by

load gradient setter the proportional channel is automaticlly switched off and the response of PRTD is purely integral.

LIMITING BY STRESS EVALUATOR When the stress evaluator is switched on, it acts so as to limit power changes to the upper and lower release margin. Limitation by means of upper release margin can lead to reduction of turbine output.Limitation by means of lower release margin can result in turbine unloading being stopped. Under certain switching conditions limitation by stress evaluator is suppresed.

KORBA SIMULATOR

270

LO

AD

CO

NT

RO

LLE

R

KORBA SIMULATOR

271

LOAD REFERENCE VALUE CONTROL

By means of the reference value control, the maximum permissible gradient for the

turboset is introduced.

This gradient is limited by means of stress evalutor or by load gradient limit. If the turboset is not synchronised with the power system, tracking to the value (PR-Pact) is effective in order that the load controller be capable of taking over control of

the turboset independent of the actual reference value which has been set.

The reference value control has PI control action, i.e. small change in reference value leads to proportional action and larger change to proportional integral action. Stress evaluator limiting is effective irrespective of the type of control action. If the load gradient limit is effective, the proportional action is switched off. The output signal of the integrator is continuously compared with P-act (actual power output) by means of an automatic circuit as long as the generator circuit breaker is not closed. This provisions facilitate smooth transition from speed control mode to load controller mode. The load controller is equipped with a ‘Load Limiter’ Module. the output of load limiter (PRMAX) limits the sum of reference values (PR) transmitted to load controller to the preselected value. Even the reduction in grid frequency cannot cause the turboset to exceed the preset power level. The output of load limiter PRMAX is adjusted by means of a setter on UCB and also indicated on the console.

If the actual power falls below the station auxiliary load, the reference value of the

power controller is tracked to match the output of the speed controller so the later assumes control of the turboset. The load controller consists of two plug-in modules :-

a) The first module accommodates the isolated grid detection and the indicating system, proportional of the load controller as well as a speed dependent correction of the load control deviation.

b) The second module houses the dynamic systems, the load controller as well as

the tracking inputs (the o/p signal of the load controller is tracked according to

KORBA SIMULATOR

272

o/p singnal of active controller controlling the load i.e. speed or pressure controller).

The signal component of CMC (Co-ordinated Master Control), PRADS which is load

reference for control device from ALDS (Automatic Load Despatch Signal) is limited by maximum and minimum load limits on CMC console and is displayed on the desk in MW. In case of fault in CMC, the PRADS signal component is switched off. The load reference and PRTD which have been matched automaticlly to PRADS while on

standby, assume control of turbo-generator. The required dependence of load on frequency in accordance with the frequency versus load charateristics is achieved by means of additional load reference value PRDF furnished by frequency unit. The unit is highly linear. the sensitivity of the

response with respect to the chage in power greid grequency is less than 5 milli Hz. The proportional component of load control may be adjusted during operation of turboset in steps of 0.5% between 2.5% and 8%. In case of failure of quartz frequency “Normal”, a monitoring cirucit automaticlly disconnets the frequency influence signal. The frequency dependent load reference (PRDF) value can be connected and

disconnected by means of a push button switch inside controller cabinet. The status of operation mode (frequncy influence on/off) is displayed on the desk. An indication displays the PRDF in MW.

All load reference values described so far are totalled in summing amplifier of the load controller. The total load reference vaue as it’s limited by load limiter is displayed on the control desk in megawatts as PR.

If the power plant together with a section of the power grid becomes isolated during load control, the load controller automaticlly switches isolated during load control, the load controller automaticlly switches to a proportional response with 5% droop. This arrangment permits safe operation of the turboset these conditions. The status of operation of load controller under these conditions described above is displayed on the control desk. TRACKING OF POWER CONTROLLER The tracking is dependent on the operating mode.

a) On load shedding to less than station auxiliary load or generator circuit breaker “off” or power controller off : (following above) speed controller controls the reference value of the power controller irrespective of the setting of pressure controller.

KORBA SIMULATOR

273

b) Generator circuit breaker “ON” and speed controller effective: (following low). The power controller operates under the speed controller; the speed controller is effective as soon as HREF power controller < HREF speed controller tracking, is released. (i.e. load controller o/p < speed controller o/p).

The operation status and faults are displayed on the desk. the EHC supplies the signal corresponding to combined reference values PRLIM PRDF to the equipment for boiler load control as an analog signal. Load dependent valve lift reference signal HRPC supplied by the load controller affects the position of the steam control valves via a selection circuit. PRESSURE CONTROLLER The signal for the actual pressure and the reference pressure furnished by boiler load control system. Pressure Influence Signal HRPC is generated by a proportional integral (PI) pressure

controller and affects directly the lift of the main steam control valves via a selection circuit. The pressure controller controls the turbine load with respect to the main steam pressure deviation, and prevents, (e.g. during a quick load increase) large pressure drop. There are two modes of operation on pressure controller INITIAL PRESSURE MODE It may be selected from control panel under certain conditions. In initial pressure mode, pressure controller tries to maintain initial pressure (Turbine inlet throttle pressure). It reduces the difference between set (reference) pressure and actual pressure to zero, by sacrificing load. The power delivered by the turbo-set is determined by the boiler capability upto a maximum of power level set by load controller. Load increase above this level is blocked. LIMIT PRESSURE MODE In the limit pressure mode the boiler storage capacity is utilised. The pressure controller influences the turbine CVs to support boiler pressure control only if a preset main steam pressure deviation is exceeded. This provision allows the load controller to handle small, quick load variation unitl pick up limit pressure control is reached. This pick up limit is seldom reached in the usual frequency supported load control mode since boiler pressure control hods pressure deviation within narrow limits. The D P is displayed on the desk.

KORBA SIMULATOR

274

If the alarm “Limit pressure engaged” is present, additional load increase is blocked by means of the control unit for reference value load. VALVE POSITION CONTROLLER The outputs from the speed controller and the load controller are compared in a MAX-MIN selectro and the output from this is again compared with the pressure controller output in a MINIMUM selector. The output from this is fed to the valve position controller. Therefore, the signals from the speed controller, load controller and pressure controllers are super imposed and selected to give an output to the valve position controller. The feed back signal from the valve lift controller (i.e. the actual valve lift signal) is derived from the differential transformers which are housed in the electro-hydraulic converter and measure the position of the power piston in the amplifier.

KORBA SIMULATOR

275

CH

AN

GE

OV

ER

SC

HE

ME

AN

D A

DM

ISSIO

N C

ON

TR

OLLE

R

KORBA SIMULATOR

276

KORBA SIMULATOR

277

KORBA SIMULATOR

278

KORBA SIMULATOR

279

KORBA SIMULATOR

280

KORBA SIMULATOR

281

PROTECTIVE DEVICES MAIN TRIP VALVES FUNCTION The function of the tripping device is to open the trip fluid circuit in the event of abnormal conditions, thereby closing the valves and thus shutting off admission of steam to the turbine. CONSTRUCTION The tripping device consists in the main of the two valves (12) that slide in the casing (11) and are loaded by the springs (5,6). The valves (12) are designed as differential pistons being forced tightly against the body assemblies (10) by the rising pressure of the fluid. Control fluid flows into the body (11) via connection a and, with a tripping device latched in ( in the position shown), into the trip fluid circuit via connection x. The trip fluid circuit leads to the stop valves and the secondary fluid circuits. Via passages drilled in the body (11) (Section A-A) fluid flows to the auxiliary trip fluid circuit which leads to the hydraulic protection devices. OPERATION When starting the unit, the valves (12) are lifted by the start up fluid (connection u 1) against the force of the springs (5,6) and forced tightly against the assemblies (10). In this way pressure is built up in the trip fluid circuit (x) and the auxiliary trip fluid circuit (x1). The pressure in the auxiliary trip fluid circuit keeps the valve in the position shown while the start up fluid drains through the start up device. Should the fluid pressure in the auxiliary trip fluid circuit drop below a specific value for any reason (e.g. by tripping of a protection device) the valves (12) move downwards due to the spring force and their own weight, thus connecting connections x and x1 with the fluid back flow c. This depressurizes the trip fluid circuit which causes the main and reheat stop valves to close. The fluid supply to the secondary fluid circuits is also shut off, thus causing the control valves to close. The two valves (12) work independently of each other so that even if one valve fails the function of the tripping device is not impaired. The limit switches (1) transmit electrical signals to the control room. EMERGENCY TRIP VALVE The emergency trip valve enables the turbine to be manually tripped from the governing rack. This valve blocks the draining of auxiliary trip fluid when it in it’s normal position. But when this valve (ball head) is pushed down, it drains the aux. Trip fluid and actuates the emergency tripping device thereby tripping the unit.

KORBA SIMULATOR

282

MAIN TRIP VALVES

SOLENOID VALVE FOR REMOTE TRIP OUT The solenoid valve is installed in the auxiliary trip fluid line to the automatic trip gear and when operated electrically causes the auxiliary trip fluid circuit to be opened and the turbine stopped the solenoid valve is remote controlled. All remote tripping commands actuates this solenoid valve only.

KORBA SIMULATOR

283

OVERSPEED TRIP The turbine is fitted with two overspeed trips which trip the turbine, when its speed exceeds rated speed by a certain value, which can be adjusted between 10 to 12 %. Each over speed trip consists of an eccentric bolt fitted in the shaft with its centre of gravity displaced from axis of rotation and held in position against centrifugal force by a spring. At the adjusted value of overspeed centrifugal force overcomes the spring force and the eccentric bolt fly out suddenly to extended positions, strike the lever catch which releases pilot valve of releasing device. The auxiliary trip fluid circuit is connected to drain thereby tripping the main trip valve, which results in turbine trip. Aux. trip fluid drains are connected back to control fluid tank and is prevented from getting mixed up with bearing lub oil. THRUST BEARING TRIP The thrust bearing trip opens the auxiliary tripping circuit in the event of axial displacement of the rotor which can be caused by excessive wear of the thrust bearing pads. This draining of the aux. trip fluid causes the turbine to trip.The mechanism for tripping is similar to that of overspeed trip releasing device. LOW VACUUM TRIP The purpose of the low vacuum trip is to operate the trip valve by draining the aux. trip fluid whenever the condenser back pressure increases beyond the permissible limits. The range in which the vacuum safety device operates can be varied by adjusting the initial tension of the spring provided in the safety device. In order to prevent aux. trip fluid from getting drained during starting and testing, (when vacuum is not there) an auxiliary pilot valve cuts off the draining of the aux. trip fluid. However the aux. pilot valve is released as soon as the speed of the machine attains certain value by connecting primary oil line to the auxiliary pilot valve and thus enabling the vacuum trip device to come into operation. Deterioration of vacuum inside condenser causes the pilot valve in the low vacuum trip to be moved from its upper position downwards by the pretension spring, resulting in depressurisation space of below right hand valve. The right hand valve is moved to the lower end position by spring, hence opening auxiliary trip fluid circuit. This causes a reduction in the pressure of fluid below differential pistons of main trip valves, and their tripping by the springs. When trip fluid circuit is open to drain, all stop and control valves close. SOLENOID VALVE FOR LOAD SHEDDING RELAY. On load shedding i.e., when the set is suddenly disconnected from the grid, the machine is intercepted by a load shedding module before it can reach trip out speed. The speed at which turbine stabilises after load shedding corresponds to governor droop and the pre-set reference speed. In general, this value is around rated speed

KORBA SIMULATOR

284

since load governing is cut in at a low load level. After load shedding, the reference speed can be decreased to rated value. One solenoid valve has been incorporated in the secondary fluid line of I.P. control valves and one in Auxiliary secondary fluid line inorder to prevent the turbine reaching trip speed in the event of the turbine suddenly loosing load. These valves are operated by a load shedding relay when the rate of change of load exceeds a certain value. The solenoid valves drain the I.P. secondary fluid and Auxiliary Secondary fluid circuits directly. Direct draining of I.P. secondary fluid circuit causes the reheat valve to close without any significant delay. HP control valves are closed due to draining of Auxiliary secondary fluid before the Hydraulic amplifier, by the second solenoid. The extraction check valves controlled by I.P. secondary fluid acting through extraction valves relay also get closed. After an adjustable time(approximately 2 sec), the solenoid valves are reclosed and secondary fluid pressure corresponding to reduced load builds up in the H.P/I.P. secondary fluid lines. SEQUENCE TRIMMING DEVICE In order to avoid excessive heating of HP exhaust, during HP/LP by pass operation a “Sequence Trimming Device” has been provided. This device comes into operation if HP exhaust pressure exceeds a pre-set value and load drops to a set valve(20%). Operation of this device further reduce the opening of IP control valves with respect to HP control valves inorder to allow more flow through HP turbine. 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 to trim device via an energised solenoid valve. When the turbine load is less than 20% and HP Exhaust pressure exceeds a preset value, the solenoid is deenergised 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 pistion of IP control valves by a lever .Upon tripping the trim device alters the spring tension of follow- up pistons of IP control valves ,draining IP secondary oil.The I.P.control valves openings are trimmed down. COLD REHEAT SWING CHECK VALVE The swing-check valve installed in the steam system between the HP cylinder and the reheater closes the cold reheat line on load shedding or tripout to prevent steam from flowing back to the HP cylinder from this line.

KORBA SIMULATOR

285

1. Cover

2. Locking Ring (Split)

3. Spacer Ring

4. Seal Ring

5. Plug

6. Shaft

7. Valve Plate Lever

8. Clack

9. Casing

10. Drainage Connection

11. Cap Nut

12. Gland Packing

13. Bearing Bush

14. Connecting Piece

SWING CHECK VALVE COLD REHEAT LINE

KORBA SIMULATOR

286

MODE OF OPERATION Depending on the secondary fluid pressure in the changeover valve controlling the swing-check valve, the latter is moved by the actuator flanged to the side of the casing (9) and connected to the shaft (6). The swing-check valve opens when the main steam stop valves have reached a specific valve lift. Below this preset point the clack (8) is returned into the steam path by the hydraulic actuator and because of the absence of steam the swing-check valve is closed by the torque of the actuator. If, when the turbine is started and the secondary fluid pressure is insufficient, the steam pressure before the swing-check valve is higher than behind the valve, the steam pressure opens the valve against the secondary fluid pressure. The position of the swing- check valve is indicated in the control room via a limit switch.

MOVING VANE SERVOMOTOR FOR CRH LINE SWITCH CHECK VALVE

MOVING VANE SERVOMOTOR FOR SWING CHECK VALVE The function of the moving vane servomotor flanged to the swing-check valve is to open or close the swing-check valve fitted in the cold reheat line. When the pilot valve operated by the transformer of the speed governor admits control fluid via connections d to the inner space of the servomotor, the adjacent connections d1 of the servomotor are at zero pressure. The control fluid then flows into the two chambers situated

KORBA SIMULATOR

287

diagonally opposite each other, thus causing the rotary vane (2) attached to the control shaft ( 16) to be turned to the stop at the segments (1 ). This rotary movement which is transmitted from the control shaft (16) to the spindle (3) of the. swing-check valve via the coupling halves (5, 6) causes the swing-check valve to close. Conversely, the swing-check valve is opened when control fluid is admitted from the connection d1. AUXILIARY VALVE (FOR EXTRACTION CHECK VALVES) FUNCTION The auxiliary valve controls the fluid supply to the extraction check valve actuators and its function is to give the check valves a signal to close in the case of a drop in load or trip-out so that steam can not flow out of the bleeder lines back to the turbine. The auxiliary valve serves several check valves (1). MODE OF OPERATION Trip fluid is admitted through connection x on the body (10) (section A-B). Secondary fluid from follow-up pistons of main control valves is admitted to the spaces above and below the valve (11) through connection b2. As the pressure above and below the valve (11) are equal under normal conditions, the valve is held in the lowest position by the force of the spring (7). With this position of the valve, the trip oil x can flow to the other valves and - as soon as these valves have been switched to the upper position by secondary fluid from follow-up piston of reheat control valves on to the changeover valves of the extraction check valves (connection x1). The check valves are then free to open. On a reduction in load, as mentioned above, the pressure above the valve (11) is reduced accordingly while the pressure below the valve is retained for a while. This is made possible by the fact that the pressure reduction below the valve is retarded by the ball (15) and the pressure in the accumulator (connection b) until the equilibrium is re-established between the pressure in the accumulator and the new pulse fluid pressure (connection b2) via the equalising passage in the cover (12). Owing to the brief differential surge, the valve (11) is forced upwards against the action of the spring (7), thus cutting off the trip fluid supply to the check valves and opening the fluid return. As a result of this, the check valves receive a closing impulse and close at reduced or reversed differential steam pressure.

The valve (16), (section C-D) are acted upon from below (connection b1) by the load-

dependent secondary fluid pressure of the control valves. If the secondary fluid

pressure exceeds the value set by adjusting the springs these valves are forced

upwards against the action of the springs and open the path for the trip fluid to the

changeover valves of the extraction check valves. The lift of the valves is limited by a

collar at their lower end. By appropriately setting the springs (14) to the valves. It can

be ascertained at which secondary fluid pressure, i.e. at which turbine load, the check valves open or receive an impulse to close.

KORBA SIMULATOR

288

If the pressure in the secondary fluid circuit drops, the valves are pushed downwards by the force of the springs and the inlet ports from the trip fluid circuit are cut off, the bleed valves thus receiving an impulse to close.The fluid in the line to the changeover valves can drain off through the opened fluid return c.

14 Spring

15 Ball

16 valve

b2 Signal fluid (Secondary fluid) from

follow-up piston of the main control

valve

C Return fluid

X Trip fluid

X1Trip fluid to change-over valve of

extraction check valve

AUXILIARY VALVE

EXTRACTION SWING CHECK VALVE Extraction swing check valves are provided to prevent the backflow of steam into the turbine from the extraction lines and feedwater heaters. Two free-swinging check valves are installed in each of the extraction lines A3, A4 and A5. In the event of flow reversal in the extraction line, the valves close automatically, whereby actuator KA01 assists the closing movement of the first check valve. The mechanical design of the swing check valves is such that they are brought into the free-swinging position by means of the trip medium pressure via actuator KA01

KORBA SIMULATOR

289

and the disc lever, and are then opened by the differential pressure present. If trip medium pressure falls, the swing check valve is moved into the steam flow by means of spring force acting via the lever, hinge pin and disc lever, and closes when differential pressure is either lowered or reversed.

AUXILIARY VALVE

The trip medium supply to actuator KA01 is controlled by extraction valve relay MAX51, AA011, changeover valves MAX51, AA048, MAX51 AA051 etc. And solenoid valves MAX51 AA028, MAX51 AA031 etc. Extraction valve relay MAX51 AA011 actuates the swing check valves in proportion to the secondary medium pressure. By suitable adjustment of the springs of valve KA02, the turbine load at which the swing check valves are released for opening or assisted in closing can be set. The opening release setting cannot be arbitrarily adjusted towards greater turbine output, as the swing check valve will open even without the release action if the steam pressure difference exerts a greater force then the closing spring. In the event of major output drops beyond the opening setting of the swing check

KORBA SIMULATOR

290

valves, there is a danger of induction of steam into the turbine form the feedwater heaters. In this case, closure of the swing check valves is assisted by a short duration close signal transmitted by valve KA01 of extraction valve relay MAX51 AA011. In normal operation, valve KA01 passes trip medium to valves KA02. After a sudden load reduction, the pressure above valve KA01 drops, whereas depressurisation beneath the valve is delayed by a check valve and the pressure in accumulator MAX 45 BB001.

EXTRACTION LINE SWING CHECK VALVE WITH ACTUATOR

Due to the resulting pressure difference, the valve moves up to interrupt the trip medium supply to the swing check valves, whose closing movement is assisted by the spring force of actuator KA01. The pressure beneath valve KA01 is slowly reduced via a flow restrictor. The valve moves back into its original position to open a way for the trip medium to release the swing check valves. By turning the handwheel on changeover valves MAX 51 AA048, MAX 51 AA051 etc., the associated swing check valves can be closed within the bounds of the effectiveness

KORBA SIMULATOR

291

of the spring. The extraction swing check valves in extraction line A4 can also be triggered by differential pressure switch LBS42 CP002. This differential pressure switch energises solenoid valves MAX 51 AA028 and MAX51 AA031 if the steam flow drops below a pre-set rate (differential pressure), thereby further assisting the closing movement of the swing check valves. The position of all swing check valves is indicated via differential transmitters - CG002A. VACUUM BREAKER FOR REDUCING THE RUNNING DOWN TIME OF THE TURBINE FUNCTION With normal shut down or tripping of the machine, the function of the vacuum breakers is to cause an increase in condenser pressure by conducting atmospheric air into the condenser together with bypass steam flowing into the condenser from the Bypass station. When the pressure in the condenser increases, the ventilation of the turbine blading 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. TOTAL VACUUM BREAKERS In special cases requiring a rapid shut down of the turboset, the total vacuum breaker is employed. ELECTRICAL CONTROL OF TOTAL VACUUM BREAKER So that the vacuum can also be broken without limitation due to condenser pressure, a manual key is provided. This key opens the vacuum breaker valve. However, it can not go into the closing position until the close key provided for closing is used. This control enables a complete equalisation of condenser and ambient pressure. AUTOMATIC CONTROL The vacuum breaker is also actuated automatically by the turbine fire protection system to shut the turboset down more quickly. It is switched back manually using the close key in this case. MODE OF OPERATION OF VACUUM BREAKER When the magnet is not excited, the solenoid valve is switched to open. The control

KORBA SIMULATOR

292

medium arising holds the vacuum breaker valve in the closed position by means of the power piston. When the pressure drops the vacuum breaker is opened by spring force OPENING PROCESS When the magnet is excited the control medium is without pressure so that the control medium in front of the solenoid valve is connected with the drain c. The piston (3) and thus the valve disc (19) are moved upwards by the force of the spring (5) (fig.). CLOSSING PROCESS When the magnet is not excited the valve is closed by the control medium arising. The pressure of the control medium a via the piston (3) presses seal ring (17) arranged in the valve disc (19) on to the valve seat (20) against the force of the compression spring (5).

VACUUM BREAKER

KORBA SIMULATOR

293

1. Lp Turbine

2. Pressure Switch

3. Solenoid Valve

4. Vacuum Breaker Valve

5. Condenser

6. Condensate Pump

7. Water Injection Valve

c Drain

d Steam From Ip Turbine

d1 Bypass Steam

k Condensate

l Atmospheric Air

FUNCTIONAL DIAGRAM OF VACUUM BREAKER

SHAFT POSITION MEASURING DEVICE The function of the shaft position-measuring device is to measure the difference between the axial expansions of the turbine casing and rotor. It is mainly used during the commissioning stages and inspections. It is not a continuous measuring device. The shaft position measuring devices can be engaged and disengaged by means of a lever provided in it. OPERATOR ACTION IN CASE ONE ESV OR IV CLOSES When stating device > 56% and all stop valves open: Close the test valve manually and open it slowly, from the valve room (first locate the test valve of the particular stop valve). When unit is running. First close the particular line control valve using ATT gearbox handle, next open the stop valve later control valve.

KORBA SIMULATOR

294

KORBA SIMULATOR

295

AUTOMATIC TURBINE TESTING SYSTEM

KORBA SIMULATOR

296

KORBA SIMULATOR

297

AUTOMATIC TURBINE TESTING SYSTEM 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 protective 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. 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 pre-testing, upon selection of a test, of the substitute devices that protect turbine during that test.

• Testing of the protective devices during normal turbine operation can only be

performed if the pretest has run without fault and protection of the turbine during testing is assured.

• Monitoring of all programme steps for execution within a predefined time.

• Interruption if the running time of any programme step is exceeded or if trip is

initiated.

• Automatic reset of test programme after a fault.

• Protection of turbine during testing provided by special test protective devices. Automatic Turbine Testing extends into trip oil piping net work 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 of the case of tripping are 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.

KORBA SIMULATOR

298

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 aux. trip fluid, closing the main trip valves. The closure of main Trip Gear drains the trip fluid, causing stop/control valves to close.

During testing, trip fluid circuit is isolated and changed over to control fluid by means of test solenoid valves and the changeover valve. This control fluid in trip circuit prevents any actual tripping of the machine. However, all alarm/annunciation gets activated as in case of an actual tripping. TEST PROCEDURE The test begins with the selection of the Protective devices subgroup. Protective device sub-group is selected by pressing the subgroup ON/OFF pushbutton. The subgroup remains in the ON position until switched off when the program has been completed. While the protective devices subgroup program is running, the other subgroups are blocked. The ON/OFF pushbutton is also used to acknowledge alarms.

KORBA SIMULATOR

299

SELECTION After the subgroup has been switched on, the protective device to be tested is selected by pressing the selection pushbutton for the individual device. A separate selection pushbutton is provided to each protective decive. Only one selection may be made at a time. Selection of a further test is possible only once all other programs have ended. TEST PUSHBUTTON The automatic test program is started by pressing the Test Pushbutton. CANCEL PUSHBOTTON This pushbotton can be used to terminate the test program running at any time and to initiate the reset program. The reset program has priority over the test program. LAMP TEST PUSHBOTTON

All the signal lamps on the control panel can be tested by pressing the Lamp Test pushbotton.

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.

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. �� Test solenoids become energised. �� Build-up of control oil pressure upstream of changeover valve is monitored. �� Test solenoids de-energised and drop of control oil pressure is monitored. �� If all steps are executed within a specified time pre-test is said to be successfull.

KORBA SIMULATOR

300

ATT OF PROTECTIVE DEVICES

KORBA SIMULATOR

301

AT

T O

F P

RO

TE

CT

IVE

DE

VIC

ES

KORBA SIMULATOR

302

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 fluid in trip fluid circuits. The test solenoids valves are again energised building up the control oil pressure upstream of changeover valve. At this moment solenoid for change over valve gets energised, draining control fluid from the bottom of the change over valve and change over valve moves to bottom position (i.e test position). With changeover valve in its test position, control fluid flows in the trip fluid piping and main trip valve gets isolated from the trip fluid header as the port for trip fluid coming from the main trip valve gets closed in the change over valve. After successful establishment of hydraulic test circuit command goes to initiate the main test, in which individual devices can be checked.

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 fluid. Due to draining of auxiliary trip fluid Main trip valves operates and trip fluid 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 fluid in aux. start-up fluid circuit to reset main trip valves and protective devices, which have tripped from their normal positions. Once they return to their normal position, trip fluid and aux. trip fluid pressure can be built-up and monitored. If fluid pressure is satisfactory then change over valve solenoid gets de-energised and change over valve moves to normal position. After this reset solenoids along with test solenoid valves de-energised, deactivating hydraulic test circuit.

HYDRAULIC TEST SIGNAL TRANSMITTERS The function of the hydraulic test signal transmitters is to activate the protective devices (with the exception of the remote trip solenoids). Each protective device has an associated test signal transmitter. For testing the overspeed trip device, the associated test signal transmitter builds up a test pressure relatively slowly and press it to the overspeed trips' for testing the low vacuum trip an air pressure signal is introduced to the device via an orifice; and for testing the thrust bearing trip, a control medium signal is passed to the test piston. The test signals to remote trip solenoids MAX52 AA001 and MAX52 AA002 turbine tester goes by itself and not by a test signal transmitter.

KORBA SIMULATOR

303

1. Compression Spring

2. Coil

3. Valve Disc

FOR THRUST BEARING TRIP

I: Test Medium To Releasing

Device

II: Drain Medium

III: Control Medium FOR LOW VACUUM TRIP

I: Vacuum To Low Vacuum Trip

II: Vacuum From Condenser

III: Air At Atmospheric Pressure

HYDRAULIC TEST SIGNAL TRANSMITTER

MAIN TRIP VALVES MAX51 AA005 and MAX51 AA006 Only one of the two main trip valves is described in the following, as they are constructionally and functionally identical. FUNCTION The function of the main trip valve is to amplify and store the hydraulic or mechanical (manually initiated local) trip signal. It must respond in the course of every successful protective device test. OPERATION Each main trip valve is kept in its operating position by auxiliary trip medium pressure. If a protective device is actuated, the auxiliary trip medium circuit is de-pressurized and the main trip valve is activated. This connects the trip medium and auxiliary trip medium circuits to drain and shuts off the control medium supply to the turbine valves. At the same time, limit switches 1 are actuated. Auxiliary start-up medium pressure forces piston 3 into its normal operating position. Control medium

KORBA SIMULATOR

304

1: Limit Switch

2: Spring

3: Piston

4: Body

I: Trip Medium

II: Aux. Trip Medium

III: Drain Medium

IV: Control Medium

V: Aux. Start-Up

Medium

MAIN TRIP VALVE

IV is then free to pass through to build up the pressure in the trip medium and the auxiliary trip medium circuits. Pressure switches MAX48 CP201 AND MAX48 CP202 monitor the auxiliary start-up medium circuit to ensure that the pressure collapses when the main trip valves latch-in nominal position. REMOTE TRIP SOLENOIDS MAX52 AA001 and MAX52 AA002 The twin electrical remote trip feature consists of the two-solenoid valves MAX52 AA001 and MAX52 AA002. One trip channel is described here, as the test procedure is the same for both. FUNCTION The function of the remote trip solenoids is to depressurize the trip medium circuit in the shortest possible time, thereby bringing main trip valves MAX51 AA005 and

KORBA SIMULATOR

305

MAX51 AA006 into their trip positions, in the event of a malfunction requiring electrical trip initiation. During normal operation the remote trip solenoid blocks the passage of auxiliary trip medium to the drain. For testing, the solenoid valve is switched over by the automatic turbine tester so that the auxiliary trip medium circuit is connected to drain. Trip initiation is monitored downstream of the main trip valves by pressure switch MAX51 CP209 and MAX52 CP211 in the auxiliary trip medium circuit. In addition, the limit switch of each main trip valve must annunciate successful completion of the test.

REMOTE TRIP SOLENOID

1. Compression Spring

2. Magnet System

3. Body

4. Vent Hole

I: Auxiliary Trip Fluid

II: Drain Medium

LATCHING-IN On successful completion of testing, solenoid valves MAX52 AA001 and MAX52 AA002 are de-energized. The reset program is then started. OVER SPEED TRIPS MAY10 AA01/MAY10 AA002 FUNCTION The two overspeed trips are provided to protect the turbine against overspeeding in the event of load coincident with failure of the speed governor. As they are particularly

KORBA SIMULATOR

306

important to the protection of the turbine, they can also be locally tested by hand during turbine operation at rated speed with the aid of overspeed trip test device (hydraulic test signal transmitter) MAX62 AA001.

OVER SPEED TRIP DEVICE

1. Turbine Shaft

2. Eccentric Shaft Fly Bolt

3. Compression Spring

4. Pawl

5. Piston

6. Limit Switch

I: Test Oil

II: Auxiliary Start-Up Medium

III: Auxiliary Trip Medium

IV: Drain Medium

OPERATION When the preset overspeed is reached, the eccentric flybolt 2 each overspeed trip activates piston 5 and limit switch annunciator 6 via pawl 4. This connects the auxiliary trip medium circuit to drain thereby de-pressurising it. The loss of auxiliary trip medium pressure causes the main trip valve to drop, which in turn causes the trip medium pressure to collapse. To activate the overspeed trip at rated speed, as the test routine performed by the automatic turbine tester requires, a specific force, equivalent to the increase in centrifugal force between rated speed and present trip overspeed, is needed. For testing, this force is exterted by the test oil pressure, acting on the head of bolt 2. On the bais of the existing defined geometry, the test oil pressure is a reproduceible measure for the trip speed, and can therefore be used to check whether the overspeed trip responds at the desired setting.

KORBA SIMULATOR

307

OVER SPEED TRIP DEVICE HTT

1. Limit Switch (Normal Position 2. Limit Switch (Test Position) 3. Valve For Test Oil 4. Actuator I: Control Medium II: Test Oil III: Auxiliary Trip Medium IV: Auxiliary Start-Up Medium V: Control Oil VI: Drain Oil

TEST SEQUENCE The test oil pressure is produced using the hydraulic test signal transmitter which is also used for manual testing First the command is given to the actuator motor to go into the trip position (down). After a cetain idling time, the test oil pressure builds up to act on the two overspeed trip bolts 2. If the two bolts are functioning correctly, they will fly outwards into the trip position when the defined pressure is reached, thereby activating the main trip valve via pawl 4, slide valve 5 and the auxiliary trip medium circuit. The two overspeed trips are monitored for actuation at the given test oil pressure by observing the two pressure switches MAX62 CP211 and MAX62 CP212 in the test oil line, and the annunciation from limit switch 6. Pressure switches MAX62 CP211 and MAX62 CP212 are preset to respond at a certain level (approx. 0.15 atm) below and above the test oil reference pressure respectively. This test reference oil pressure is determined empirically during commissioning and entered in the operating log. Limit siwtch 6 must respond within the pressure range between the settings of pressure switches MAX62 CP211 and MAX62 CP212. A slow build-up of pressure is required for this operation, which is why a relatively long monitoring period, equivalent to the running time of the actuator, has to be selected. Premature response of the overspeed trips is annunciated.

KORBA SIMULATOR

308

LATCHING-IN Once the trip has been initiated, the actuator of the hydraulic test signal transmitter is driven back until the integral limit switch annunciates that normal position has been reached. In addition, monitoring must be continued until the test oil pressure at pressure switch MAX62 CP213 is less than 0.1 atm. This double check-back of the hydraulic test signal transmitter having returned to normal position ensures that, after completion of testing, the overspeed at which the turbine will trip is not reduced due to test oil pressure remaining effective and that the overspeed trip will not be set off prematurely in the event of load reduction. While test oil pressure is being dispersed, the two overspeed bolts spring back into their normal positions at a pressure well above 0.5 atm. Subsequently by, piston 5 is brought back into its nominal position by pressure of auxiliary start-up medium II and latched in with pawl 4. At the same time, piston 5 shuts off drain channel IV, so auxiliary trip medium III can build up pressure. Once this has been done, the auxiliary start-up medium can be depressurized. LOW VACUUM TRIP MAG01 AA011 FUNCTION The function of the low vacuum trip is to operate the main trip valve during normal operation, if the vacuum in 4, the turbine condenser is too weak for condensation to be properly effected. OPERATION In each trip device, compression spring 3, set to a specific tension, pushes downwards against diaphragm 4, the top side 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 7 is thereby dispersed and the auxiliary trip medium circuit is connected to drain. The resultant depressurization of the auxiliary trip medium circuit actuates main trip valves MAX51 AA005 and MAX51 AA006, thereby closing all turbine valves. TEST SEQUENCE First, test signaller (solenoid valve) MAG01 AA201, fitted in the vacuum carrying signal line, is energized. This blocks off the vacuum line and simultaneously connects the space above diaphragm 4 to the atmosphere, so that air is free to flow in via an orifice (connection II) to weaken the vacuum. Compression spring 3 presses down valve 6 to

KORBA SIMULATOR

309

connect the auxiliary trip medium circuit to drain via valve 7 when the preset limit is reached The low vacuum trip is monitored for operation within the specified vacuum range by observing pressure switches MAG01 CP202 and MAG01 CP201.

LOW VACUUM TRIP DEVICE

1. Adjusting

Piston 2. Stem

(Adjustable) 3. Compression

Spring 4. Diaphragm 5. Limit Switch 6. Valve 7. Valve

I. Primary Oil II. Vacuum III. Atmospheric

Pressure IV. Auxiliary Trip

Medium V. Drain Medium VI. Control

Medium

LATCHING-IN

When test signal transmitter MAG01 AA201 has been de-energized and the connection

between low vacuum trip and the condenser re-established, vacuum builds up again

above diaphragm 4. Valve 6 moves into its upper end position, thereby opening the

passage for the control medium to flow to valve 7. When valve 7 is in its upper end

position, the auxiliary trip medium circuit is closed again. Restoration of normal

operating configuration is annunciated by the limit switch of the low vacuum trip and

by pressure switch MAG01 CP201.

KORBA SIMULATOR

310

THRUST BEARING TRIP DEVICE

1. Shaft

2. Tripping Cam

3. Pawl

4. Test Piston

5. Tension Spring

6. Valve Piston

7. Compression Spring

8. Limit Switch

I: Auxiliary Trip Medium

II: Auxiliary Start-Up Medium

III: Drain Medium

IV: Test Medium

KORBA SIMULATOR

311

THRUST BEARING TRIP MAY10 AA011 FUNCTION 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 medium circuit in the shortest possible time, thereby tripping the turbine. OPERATION The two rows of tripping cams 2 which are arranged on opposite sides of turbine shaft 1 have a specific clearance, equivalent to the permissible shaft displacement, relative to pawl 3 of the thrust bearing trip. If the axial displacement of the shaft exceeds the permissible limit, the cams engage pawl 3, which releases piston 6 to de-pressurize the auxiliary trip medium circuit and at the same time to actuate limit switch 8. TEST SEQUENCE To test the thrust bearing trip, the associated hydraulic test signal transmitter MAX61 AA202 is energized. Test medium IV is then free to pass to test piston 4, which then deflects pawl 3 against the force of spring 5. The combined force of the auxiliary trip medium and of compression spring 7 drives piston 6 into its trip position. The pressure of test medium IV is monitored by pressure switch MAX61 CP211. LATCHING-IN After the pressure of test medium IV has dropped again, the thrust bearing trip is returned to its normal operating position by applying the pressure of start-up auxiliary medium II to valve piston 6, thus moving the piston against the force of compression spring 7. Thereby pawl 3 latches in and holds valve piston blocks off drain III. Once this has been done, the auxiliary start-up medium is depressurized again. RESET SOLENOIDS MAX48 AA201 AND MAX48 AA 202 FUNCTION The function of the reset solenoids is to restore the tripped protective devices to their normal operating positions during the ATT reset program. OPERATION The reset solenoids are two 3-way solenoid valves (2 ports open at any time), fitted in the auxiliary start-up medium line. Both solenoid valves are energized in the course of the reset program conducted after each subtest, so that auxiliary start-up medium

KORBA SIMULATOR

312

line II is supplied with control medium III. The control medium pressure forces all protective devices back into their normal operating positions, and the trip medium and auxiliary trip medium pressure can build up again. When the protective devices have latched in again, reset solenoid MAX48 AA201 is de-energized first to shut off the control medium supply through this valve. Dispersion of the auxiliary start-up medium pressure is monitored by pressure switch MAX48 CP201. The second reset solenoid MAX48 AA202 is then de-energized to disperse the pressure between the two solenoid valves. This is monitored by pressure switch MAX48 CP202. The use of two reset solenoids ensures that main trip valves MAX 51 AA005 and MAX 51 AA006 will always be sure to be actuated if either one of the two reset solenoids is deenergized.

RESET SOLENOID

1. Compression Spring

2. Coil

3. Valve Disc

I: Auxiliary Start-Up Medium

II: Auxiliary Start-Up MediumTo

Protective

Devices

III: Control Medium

ATOMATIC TURBINE TESTER FOR STOP AND CONTROL VALVES GENERAL The stop valves and the control valves of the turbine are the final control elements governed by the protective the final control elements governed by the protective devices and it is therefore equally important that these, as well as the protective devices, should function reliably. The testing of these valves in conjunction with testing of the protective devices, ensures that all elements which must respond on turbine trip are tested for their ability to function reliably.

KORBA SIMULATOR

313

Each stop valve is tested together with its associated control valve. The automatic turbine tester is designed so that only one valve assembly may be selected and tested at any time. TEST REQUIREMENTS To avoid turbine output changes and initial pressure variations due to the closing of the tested control valve during testing, the electro-hydraulic turbine controller must be in operation prior to testing. For the same reason, the closing time of the control valves is relatively long. To enable initial pressure to be maintained constant, testing is only permissible when the turbine output is below a certain value. SPECIAL CONDITIONS DURING TESTING During testing one of the control valves MAA10 to 40 AA002 or MAB10 to 40 AA002 is closed completely by means of a motor-operated positioner - AA002M acting on relay piston KA06 parallel to pilot valve KA05. This results in a closing movement simulating that which occurs when the associated secondary pressure drops. This constant slow closing movement is also necessary in order to enable the associated controller to keep the output or initial pressure constant. Thus the conditions for actuation of the valve are the same during testing as during normal actuation by the controller.

The stop valves, which are held in the open position by trip medium pressure during

normal operation, are subjected to exactly the same hydraulic conditions during

testing as would be the case in the event of actual turbine trip, as the action of the

protective devices is simulated by solenoid valve MAX61 AA211 to 214 or MAX61

AA221 to 224, respectively. The steam side conditions during testing are somewhat

more severe than during actual trip, as the pressure downstream of the stop valve

connot drop off during closure because the control valve is closed. This means that the

steam pressure acting against the spring closure force (steam lift) is greater than is the

event of normal trip. The automatic turbine tester intervenes in the medium circuits normally used to control the valves and uses only trip medium both for operation of test valves MAX47 AA011 to 014 and MAX47 AA021 to 024 and for resetting and opening of the stop valves. Thus closure of the valves cannot be impeded in the event of a trip during testing, regardless of the stage which the test has reached. This also applies to the control valves, as the ATT does not interrupt the secondary medium circuit and the

KORBA SIMULATOR

314

secondary medium can thus be depressurized in the normal manner in the even of a trip.

ATT OF STOP/CONTROL VALVES

KORBA SIMULATOR

315

FEATURES OF THE AUTOMATIC TURBINE TESTER

• The automatic tester is distinguished by the following features:

KORBA SIMULATOR

316

• Individual testing of each valve assembly

• Monitoring of all program steps for execution within a certain time.

• Automatic reset of the testing program after a fault.

Protection of the turbine during testing provided by special test protective devices. TEST SELECTION UNITS There are 4 combined main stop and control valves and 4 combined reheat stop and control valves, each of which is tested as a separate unit and has a separate selection push button in the ATT control panel. They are:

• Selection 1: Main stop and control valve, • Selection 2: Main stop and control valve, • Selection 3: Main stop and control valve, • Selection 4: Main stop and control valve, • Selection 5: Reheat stop and control valve, • Selection 6: Reheat stop and control valve, • Selection 7: Reheat stop and control valve, • Selection 8: Reheat stop and control valve,

TEST PROCEDURE: START OF TESTING The test begins with the selection of the valve test subgroup. This is performed by pressing the subgroup ON/OFF pushbutton. The subgroup remains in the ON postion until switched off when the program has been completed. While the valve test subgroup program is running, the other subgroups are blocked. The On/Off pushbutton is also used to acknowledge alarms. SELECTION If the test requirements have been fulfilled and the balve test subgroup switched on, the valve assembly (e.g. main stop and control valve, to be tested is selected by pressing the selection pushbutton for the individual valve assembly. A separate selection pushbutton is provided for each valve assembly. Only one selection may be made at a time. Selection of further test is possible only once all other programs have ended. TEST PUSHBUTTON

KORBA SIMULATOR

317

The automatic program is started by pressing the Test pushbutton of the valve test program tile. CANCEL PUSHBUTTON All the signal lamps on the control panel can be tested by pressing the Lamp Test pushbutton. CLOSURE OF CONTROL VALVE If all the test requirements have been fulfilled and the selection and Test pushbottons pressed, control valve MAA10 to 40 AA002 or MAB10 to 40 AA002 of the valve assembly selected is closed by means of its associated valve test positioner (test motor, -AA002M). Operation of positioner -AA002M is continued until limit switches CG002C on the control valve and -AA002 MS72 and -AA002 MS73 on the actuator are actuated to annunciate, with a slight delay to achieve a ceratin overtravel, that the control valve being tested is in closed position.

During this time, the controllers compensate for the effects of closure of the valve

being tested on the initial pressure or turbine output by opening the other control

valves. The running time for closure of the control valve is monitored. If the control valve is functioning properly, it will close within the pre-set running time. CLOSURE OF STOP VALVE If the control valve has closed properly, solenoid valve MAX61 AA211 to 214 or MAX221 to 224 is energized. This allows trip medium to flow to the space below changeover slide valve MAX61 AA011 to 014 or MAX61 AA021 to 024, which moves into its upper end position and connects the space below piston disc KA02 with the drain. The pressure in this space drops rapidly and is monitored by the pressure switch MAX51 CP223 CP223, 228, 233, 238 or MAX51 CP248, 253, 258, respectively. When the pressure at this pressure switch has dropped slightly below the break-away pressure of piston disc KA02, monitoring of the stop valve closure time starts. Limit switch - CG001E annunciated entry of the valve into its closed position, thus making it possible to monitor the valve closing action for completion within the maximum running time.

OPENING OF STOP VALVE

Next, solenoid valve MAX47 AA211 to 214 or MAX47 AA221 to 224 is energized (test

position) and trip medium is admitted to the control surface of the piston in test valve

KORBA SIMULATOR

318

MAX47 AA011 to 014 or MAX47 AA021 to 024. The piston moves into its lower end

position against the spring force, thus permitting trip medium to flow to the space

above piston KA01 of the stop valve. This piston is forced downwards by the pressure

of the medium, thereby tensioning the spring between piston KA01 is relatively low,

being equal to the spring force acting against it. The spontaneous pressure rise when

piston KA01 has made contact with piston disc KA02, and thus on completion of the

spring tensioning action, is detected by pressure switch MAX51 CP222, 227, 232, 237

or MAX51 CP242, 247, 252, 257. If all conditions are fulfilled within this relatively

long monitoring period, solenoid valve MAX61 AA211 to 214 or MAX61 AA221 to 224

is de-energized (operating position), so that trip medium is once again able to flow to

test valve MAX47 AA011 to 014 or MAX47 AA021 to 024 and the drain is blocked off

again. The build-up of trip medium pressure monitored by pressure switch MAX51

CP221, 226, 231, 236 or MAX51 CP241, 246, 251, 256.

When the pressure is sufficiently high, the stop valve is opened by de-energizing

solenoid valve MAX47 AA211 to 214 or MAX47 AA221 to 224 (operating position). Test

valve MAX47 AA011 to 014 and MAX47 AA021 to 024 switches over, admitting trip

medium to the underside of piston disc KA02 and, after a certain amount of further

travel, slowly connects the space above piston KA01 with the drain. The resultsant

pressure difference causes the tensioned piston relay to open the stop valve. As soon

as open position is reached, the full trip medium pressure builds up. This is

monitored by pressure switch MAX51 CP223, 228, 233, 238 or MAX51 CP243, 248,

253, 258 and by limit switch - CG001D. Testing of the stop valve is now completed. RE-OPENING OF CONTROL VALVE If the conditions are fulfilled within the specified monitoring period, the control valve is re-opened. The motor of positioner--AA002M is operated in the opening direction. Positioner -AA002M moves the control valve into its original position in the reverse sequence to the closing action. Again the initial pressure and output are kept constant by the appropriate controller. Operation of positioner -AA002M is continued until, after a certain amount of overtravel, it has positively ceased to influence the controller. This position is detected by limit switch -AA002 MS61 or -AA002 MS62. If the control valve is functioning properly, it will open within the pre-set running time. CANCELLATION OF SELECTION On conclusion of testing of each valve assembly, the selection is automatically canelled and the program shut down.

KORBA SIMULATOR

319

INTERRUPTION DUE TO RUNNING TIME EXCEEDED The reset program is automatically initiated if the running time for any step is the test program is exceeded. If any running time is exceeded during the reset program, the program is halted. In either case, the alarms Fail signal and Time Overrun are generated. If the Fault in ATT alarm is displayed, the fault lies in the automatic tester itself. INTERRUPTION DUE TO TURBINE TRIP If electrical turbine trip is initiated during testing, all solonoid valves are de-energized and positioner -AA002M is returned to its extreme position and the program cancelled. All equipment associated with the automatic turbine tester is automatically returned to its normal position.

KORBA SIMULATOR

320

KORBA SIMULATOR

321

KORBA SIMULATOR

322

TURBINE STRESS EVALUATOR

KORBA SIMULATOR

323

KORBA SIMULATOR

324

TURBINE STRESS EVALUATOR Steam parameter varies with the condition throughout the operating range with every change (start up, loading, shutdown) of the turbine, and in turn turbine metal is subjected to those temp changes. The resulting �T in the turbine material is a measure of thermal stress subjected to that part. As thermal stress becomes major consideration from turbine side, it is to be ensured that turbine is never subjected to undesirable thermal stress. Differential Temperature in the material should always be kept within the permissible limits. The optimum balance between longevity on the one hand and max flexibility of operation on the other is achieved when the permissible range of material stress can be utilised to the full. 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. INPUTS TO THE TSE

Actual Speed (Hall Probe) : 0 -3600 rpm

Actual Load : 0 -600 MW

Temperatures Of Turbine Metal Parts : MSV, MCV, HPC. HPS, IPS

TSE gets inputs from 5 points of turbine to compute limits for operation: 1. Main ESV Temp. (Surface and midwall.) 2. MCV Temp. (Surface and midwall.) 3. HP Shaft Temp. (Surface and midwall.) 4. HP Casing Temp. (Surface and midwall.) 5. IP Shaft Temp. (Surface and midwall.) Temperature at various points are measured at surface (96% depth) and inside (54%) with the help of thermocouples placed at proper places. In case of HP shaft and IP shaft, temperature is measured at the casing where thermal behavior of shaft and casing is supposed to be same. The mean internal (mid metal) shaft temperature can be calculated with an adequate degree of accuracy by means of the following mathematical equation.

KORBA SIMULATOR

325

Tm = Ts [ 1- (0.692 e -t/T1 + 0.131 e -t/T2 + 0.177 e -t/Tk ) ]

Where, Ts : Surface Temperature T1 : 2408.31

Tm : Mid metal Temperature T2 : 457.08

t : Time in minutes Tk : 56.62

Time constants

Various constants used in the above equation are derived from the shaft diameter and thermal diffusivity of the rotor material. CALCULATION CIRCUIT The milli-volt output from thermocouple is fed into the signal conditioning cabinet where the transducers give out 4-20 mA signals as temperature signals. For calculation purpose we have one analog computer located in the controller cabinet having five computing channels for above five inputs. Each computing channels determines the temperature difference (∆Ta) between surface and mid metal temperature. The thermal stress is proportional to this temperature difference. The calculated temperature difference is compared against the permissible temperature 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 + ve side, we get upper margin and the –ve side we get lower margin. The smallest of the respective upper and lower temperature Margins computed for admission and turbine areas are selected for display on the TSE indicator and used for further processing. TSE DISPLAY 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 Load - Two 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

KORBA SIMULATOR

326

TSE

BL

OC

K D

IAG

RA

M

KORBA SIMULATOR

327

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. 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.

KORBA SIMULATOR

328

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. 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).

KORBA SIMULATOR

329

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.

KORBA SIMULATOR

330

IMPORTANCE OF TSE MARGINS The temperature margin is a measure of the degree of thermal stress, which a turbo set, is subjected to, during speed rising before synchronization. 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 limits. This condition is indicated by either of upper or lower red disc reaching horizontal position.

If the upper load or speed margin is consumed then the following methods can be adopted to restore the lost margin.

• Avoidance of further loading of turbine. • Reduction of steam temperatures in case boiler firing rate had been rapid. • Soaking the machine for sufficient time period.

If the lower margin gets consumed then the following methods can be adopted

• Stopping of further unloading & soaking the turbine. • Increase the steam temperatures.

It is worthwhile to mention here that before synchronization turbine should have temperature margin more than 300 K available so that minimum load on the set can be achieved immediately after synchronization.

OUTPUTS

ATRS :

EHC : + 30 DEG.K

CMC : + 30 DEG.K

TSE TEMP. RECORDE : 0 - 600 DEG.C.

TSE MARGIN RECORDER : + 150 DEG.K.

TSE INDICATOR : + 150 DEG.K., 0 TO 600 MW

TO ATRS All metal temperature criteria come from curve generators/LVRs. Inputs to these are from the same thermocouple that are used for TSE

KORBA SIMULATOR

331

• In SGC Turbine step 14 TSE Test programme block command is given.

• In SGC Turbine step 15 TSE margin more than 30O K. criteria is checked.

• If TSE influence is made off, SGC Turbine program goes OFF.

• TSE fault does not switch off ATRS SGC Turbine program. In case of fault last

margin is frozen so that rolling does not get affected till 2850 rpm. EHC Only temperature margins are taken and not load margins. SPEED CONTROLLER: Only upper margin is used. Lower margin is not used, as coasting down is natural. TSE margins determine the gradient at which NRTD varies. LOAD CONTROLLER: Both lower and upper margins used. In set point controller, these margins determine the gradient at which PRTD varies –

If TSE influence is OFF, 10 volt is fed at TSE margin input to minimum gate so that

UCB gradient controls the P rtd rate. 10V input is used for clamping the max gradient allowable. It is not from UCB but from EHC panel.Negative upper temperature margin can unload the machine but loading the machine is left to desk engineer. STOP REFERENCE (LOAD/SPEED): TSE fault is one of the conditions for stop reference. In case of TSE fault TSE influence is to be made off and then reset the fault. Then switch on TSE influence otherwise the set/reset memory may not reset properly.

KORBA SIMULATOR

332

24 V SUPPLY OFF TO TSE PANEL: Discs will close on TSE indicator. All lamps in TSE indicator will go off. TSE fault occurs. TSE release will be absent for EHC and load ref. stop will come. TSE INDICATOR SUPPLY UCB OFF : TSE discs will close. All lamps will go OFF in the TSE indicator. TSE TEST Release should be present from EHC, CMC and ATRS. EHC, CMC release will come if the controllers are balanced i.e. there is no variation between actual and set values. Testing can be dome if TSE influence is OFF in the absence of these releases. ATRS SGC turbine programme step-14 output should be absent; otherwise, TSE testing will be blocked. TSE test is carried out in order to check the healthiness of various computing channel and display channels, which are coming to TSE display. Testing is possible only when enable signal from EHC is present. For testing known input data can be introduced for each channel to give rise to display of certain predetermined results. If TSE is functioning correctly, the indicator must show specific values for each computing channel. If there is deviation from the tolerance test values, it is probable that there is a fault in the evaluator. TSE influence switching off facility is there to be used under fault condition. For healthiness of computing channels dynamic monitoring system is also active. Dynamic monitoring system is based on the principle of detecting change as a function of time. Each channel generates the gradient for its curves. These circuits keep watch on temperature requisition and nature of kompensograph ensures if a pre-set gradient is exceeded i.e. characteristic gradient is noticed, the dynamic monitoring loop responds and associated lamplights up with fault indication. This facilitates rapid localization of defective computing channel. Test program is available for display not for kompensograph. Power supply is separately available for display and kompensograph. In case of loss of supply for display, kompensograph will not affected. ALARMS

1. On exceeding the limits computed by TSE, Margin spent alarm is initiated.

2. TSE fault alarms is initiated on following condition, shown on centre window of the TSE display in red colour

KORBA SIMULATOR

333

• Enabling signal for test • Transducer fault • Fault detected by dynamic monitoring • Hardware fault

DISPLAY RESULTS FROM THE TEST PROGRAM

Main steam stop valve (MSV) Top screen 30 + 1 k

Bottom screen 70 + 1 k

Main steam control valve (MCV) Top screen 61 + 1 k

Bottom screen 35 + 1 k

HP Turbine Casing (HPC)

Variable-speed range Top screen 29 + 1 k

Bottom screen 43 + 1 k

Power output range Top screen 503 + 6 mw

Actual power 401 + 6 mw

Bottom screen 104 + 6 mw

HP Turbine Shaft (HPS) Top screen 58 + 1 k

Variable Speed range Top screen 28 + 1 k

Power output range Top screen 554 + 6 mw

Actual power 400 + 6 mw

Bottom Screen 206 + 6 mw

IP Turbine Shaft (IPS)

Variable speed range Top screen 96 + 1 k

Bottom screen 56 + 1 k

Power output range Top screen 451 + 6 mw

Actual power 401 + 6 mw

Bottom screen 152 + 6 mw

KORBA SIMULATOR

334

KORBA SIMULATOR

335

KORBA SIMULATOR

336

GENERATOR AND

GENERATOR AUXILIARIES

KORBA SIMULATOR

337

KORBA SIMULATOR

338

GENERATOR & ITS AUXILIARIES GENERAL DESIGN FEATURES

Make : BHEL

Type : THDF 115/59

Code : IEC 34-1, VDE 0530

Cooling ,stator winding : Directly water cooled

Stator core ,rotor : Directly hydrogen cooled.

Rating

Apparent power : 588 MVA

Active power : 500 MW

Power factor : 0.85(LAG)

Terminal voltage : 21 KV

Permissible variation in voltage : +5%

Speed/Frequency/Hz : 3000/50

Stator current : 16200

Hydrogen pressure : 4 Kg/Cm2

Short circuit Ratio : 0.48

Field Current(calculated value) : 4040 A

Class and Type of Insulation : MICALASTIC (similar to class F)

No. of terminals brought out : 6

Resistance in Ohms at 20OC

: U-X 0.0014132

: V-Y 0.0014145

Stator Winding between terminals

: W-Z 0.0014132

Rotor Winding : F1-F2 0.0672

KORBA SIMULATOR

339

GENERAL DESCRIPTION The two-pole generator uses direct water cooling for the stator winding, phase connectors and bushings and direct hydrogen cooling for the rotor winding. The losses in the remaining generator components, such as iron losses windage losses and stray losses, are also dissipated through hydrogen. The generator frame is pressure-resistant and gas tight and equipped with one stator end shield on each side. The hydrogen coolers are arranged vertically inside the turbine end stator end shield. The generator consists of the following components : Stator Stator frame End shields Stator core Stator winding

Main Exciter :

Active Power : 3780 KW

Current : 6300 A

Voltage : 600V

Pilot Exciter

Apparent power : 65 KVA

Current : 195 A

Voltage : 220 V(1+10%)

Frequency : 400 Hz

Torque, Critical Speeds

Maximum short circuit torque of stator at line to

line single phase short circuit

: 1488 kpm

Moment of inertia of generator shaft : 10,000kgm2

Critical speed (calculated)

nk1 : 14.4 rps(V-GEN)

nk2 : 30.1rps(V-EXC)

nk3 : 39.8rps(S-GEN)

KORBA SIMULATOR

340

Hydrogen coolers Rotor Rotor shaft Rotor Winding Rotor retaining rings Field connections Bearings Shaft seals The following additional auxiliary systems are required for generator operation :- Oil system

Gas system

Primary water system

Excitation system

COOLING SYSTEM

The heat losses arising in the generator interior are dissipated to the secondary

coolant (raw water, condensate etc.) through hydrogen and primary water.Direct

cooling essentially eliminates hot spots and differential temperatures between adjacent

components which could result in mechanical stress, particularly to the copper

conductors, insulation rotor body and stator core.

HYDROGEN COOLING CIRCUIT :

The hydrogen is circulated in the generator interior in a closed circuit by one multi-

stage axial-flow fan arranged on the rotor at the turbine end. Hot gas is drawn by the

fan from the air gap and delivered to the coolers, where it is recooled and then divided

into three flow path after each cooler.

FLOW PATH I Flow path I is directed into the rotor at the turbine end below the fan hub for cooling

of the turbine end half of the rotor.

KORBA SIMULATOR

341

KORBA SIMULATOR

342

FLOW PATH II Flow path II is directed form the coolers to the individual frame compartments for cooling of the stator core. FLOW PATH III Flow path III is directed to the stator end winding space at the exciter end through guide ducts in the frame for cooling of the exciter end half of the rotor and of the core end portions. The three flows mix in the air gap. The gas is then returned to the coolers via the axial-flow fan. The cooling water flow through the hydrogen coolers should be automatically controlled to maintain a uniform generator temperature level for various loads, and cold water temperatures. COOLING OF ROTOR For direct cooling of the rotor winding, cold gas is directed to the rotor end windings at the turbine and exciter ends. The rotor winding is symmetrical relative to the generator centre line and pole axis. Each oil quarter is divided into two cooling zones. The first cooling zone consists of the rotor end winding and the second one of winding portion between the rotor body end and the mid point of the rotor. Cold gas is directed to each cooling zone through separate openings directly before the rotor body end. The hydrogen flows through each individual conductor in closed cooling ducts. The heat removal capacity is selected such that approximately identical temperatures are obtained for all conductors. The gas of the first cooling zone is discharged from the coils at the pole centre into a collecting compartment within the pole area below the end winding. From there the hot gas passes into the air gap through pole face slots at the end of the rotor body. The hot gas of the second cooling zone is discharged into the air gap at mid-length of the rotor body through radial openings in the hollow conductors and wedges. COOLING OF STATOR CORE For cooling of the stator core, cold gas is admitted to the individual frame compartments via separated cooling gas ducts. From these frame compartments the gas then flows into the air gap through slots in the core where it absorbs the heat from the core. To dissipate the higher losses in the core ends, the cooling gas slots are closely spaced in the core end sections to ensure effective cooling. These ventilating ducts are supplied with cooling gas directly from the end winding space. Another flow path is directed from the stator end winding space past the clamping fingers between the pressure plated and core end section into the air gap. A further flow path passes into the air gap along either side of the flux shield.

KORBA SIMULATOR

343

All the flows mix in the air gap and cool the rotor body and stator core surfaces. The gas is then returned to the coolers via the axial-flow fan. To ensure that the cold gas directed to the exciter end cannot be directly discharged into the air gap, an air gap choke is arranged with in the range of the stator end winding cover and the rotor retaining ring at the exciter end. PRIMARY COOLING WATER CIRCUIT IN THE GENERATOR The treated water used for cooling of the stator winding phase connectors and bushing is designated as primary water in order to distinguish it from the secondary coolant (raw water, condensate, etc.). The primary water is circulated in a closed circuit and dissipates the absorbed heat to the secondary cooling water in the primary water cooler. The pump is supplied with hot primary water from the primary water tank and delivers the water to the generator via the coolers. The cooled water flow is divided into two flow paths as described in the following paragraphs. FLOW PATH I Flow path I cools the stator windings. This flow path first passes to a water manifold on the exciter end of the generator and from there to the stator bars via insulated hoses. Each individual bar is connected to the manifold by a separate hose. Inside the bars the cooling water flows through hollow strands. At the turbine end, the water is passed through similar hoses to another water manifold and then returned to the primary water tank. Since a single pass water flow through the stator is used, only a minimum temperature rise is obtained for both the coolant and the bars. Relative movements due to different thermal expansions between the top and bottom bars are thus minimised. FLOW PATH II Flow path II cools the phase connectors and the bushings. The bushings and phase connectors consists of thick walled copper tubes through which the cooling water is circulated. The six bushings and the phase connectors arranged in a circle around the stator end winding are hydraulically interconnected. The secondary water flow through the primary water cooler should be controlled automatically to maintain a uniform average generator temperature level for various loads and cold water temperatures. STATOR FRAME The stator frame consists of a cylindrical centre section and two end shield which are gas tight and pressure resistant. The stator end shields are joined and sealed to the stator frame with an O-ring and bolted flange connections. The stator frame accommodates the electrically active parts of the stator, i.e. the stator core and the stator windings. Both the gas ducts and a large number of welded circular ribs provide for the rigidity of the stator frame. Ring shaped supports for resilient core suspension are arranged between the circular ribs. The generator cooler is subdivided into cooler sections arranged vertically in the

KORBA SIMULATOR

344

turbine side stator end shield. In addition, the stator end shields contain the shaft seal and bearing components. Feet are welded to the stator frame and end shields to support the stator on the foundation. The stator is firmly connected to the foundation with anchor bolts through the feet. STATOR CORE The stator core is stacked from insulated electrical sheet steel laminations and mounted in supporting rings over insulated dovetailed guide bars. Axial compression of the stator core is obtained by clamping fingers, pressure plates and non-magnetic trough type clamping bolts which are insulated from the core. The supporting rings form part of an inner frame cage. This cage is suspended in the outer frame by a large number of separate flat springs which are tangentially arranged on the circumference in sets of three springs each, i.e. two vertical supporting springs on both sides of the core and one horizontal stabilising spring below the core. The springs are so arranged and tuned that forced vibrations of the core resulting from the magnetic field will not be transmitted to the frame and foundation. The pressure plates and end portions of the stator core are effectively shielded against stray magnetic fields. The flux shields are cooled by a flow of hydrogen gas directly over the assembly. STATOR WINDING Stator bars, phase connectors and bushings are designed for direct water cooling. In order to minimise the stray losses, the bars are composed of separately insulated strands which are transposed by 540O in the slot portion and bonded together with epoxy resins in heated moulds. After bending, the end turns are likewise bonded together with backed synthetic resin fillers. The bars consists of hollow and solid strands distributed over the entire bar cross section so that good heat dissipation is ensured. At the bar ends, all the solid strands are jointly brazed into a connecting sleeve and the hollow strands into a water box from which the cooling water enters and exits via teflon insulating hoses connected to the annular manifolds. The electrical connection between top and bottom bars is made by a bolted connection at the connecting sleeve. The water manifolds are insulated from the stator frame, permitting the insulation resistance of the water-filled winding to be measured. During operation, the water manifolds are grounded. MICALASTIC HIGH - VOLTAGE INSULATION High-voltage insulation is provided according to the proven Micalastic system. With this insulating system, several half-overlapped continuous layers of mica tape are applied to the bars. The mica tape is built up from larger mica splitting which are sandwiched between two polyester backed fabric layers with epoxy as an adhesive. The number of layers, i.e., the thickness of the insulation depends on the machine voltage. The bars are dried under vacuum and impregnated with epoxy resin which has very good penetration properties due to its low viscosity. After impregnation

KORBA SIMULATOR

345

under vacuum, the bars are subjected to pressure, with nitrogen being used as pressurising medium. The impregnated bars are formed to the required shape in moulds and cured in an oven at high temperature. The high-voltage insulation obtained is nearly void - free and is characterised by its excellent electrical, mechanical and thermal properties in addition to being fully water proof and oil - resistant. To minimize corona discharges between the insulation and the slot wall, a final coat of semiconducting varnish is applied to the surfaces of all bars within the slot range. In addition, all bars are provided with an end corona protection to control the electric field at the transition from the slot to the end winding and to prevent the formation of creepage spark concentrations. BAR SUPPORT SYSTEM To protect the stator winding against the effects of magnetic forces due to load and to ensure permanent firm seating of the bars in the slots during operation, the bars are inserted with a side ripple spring, a hot-curing slot bottom equalising strip, and a top ripple located beneath the slot wedge. The gaps between the bars in the stator end windings are completely filled with insulating material and cured after installation. For radial support, the end windings are clamped to a rigid support ring of insulating material which in turn is fully supported by the frame. Hot-curing conforming fillers arranged between the stator bars and the support ring ensure a firm support of each individual bar against the support ring. The bars are clamped to the support ring with pressure plates held by clamping bolts made from a high-strength insulating material. The support ring is free to move axially within the stator frame so that movements of the winding due to thermal expansions are not restricted. The stator winding connections are brought out to six bushings located in a compartment of welded non - magnetic steel below the generator at the exciter end. Current transformers for metering and relaying purposes can be mounted on the bushings. ROTOR Rotor Shaft The high mechanical stresses resulting from the centrifugal forces and short-circuit torque call for a high quality heat-treated steel. Therefore, the rotor shaft is forged from a vacuum cast steel ingot. Comprehensive tests ensure adherence to the specified mechanical and magnetic properties as well as homogeneous forging. The rotor shaft consists of an electrically active portion, the so-called rotor body, and the two shaft journals. Integrally forged flange couplings to connect the rotor to the turbine and exciter are located outboard of the bearings. Approximately two-thirds of the rotor body circumference is provided with longitudinal slots which hold the field winding. Slot pitch is selected so that the two solid poles are displaced by 180 deg.

KORBA SIMULATOR

346

Due to the non-uniform slot distribution on the circumference, different moments of inertia are obtained in the main axis of the rotor. This is turn causes oscillating shaft deflections at twice the system frequency. To reduce these vibrations, the deflections in the direction of the pole axis and the neutral axis are compensated by transverse slotting of the pole. After completion, the rotor is balanced in various planes at different speeds and then subjected to an overspeed test at 120 % of rated speed for two minutes. The solid poles are also provided with additional longitudinal slots to hold the copper bars of the damper winding. The rotor wedges act as a damper winding in the area of the winding slots. COOLING OF ROTOR WINDING Each turn is subdivided into eight parallel cooling zones. One cooling zone includes the slots from the centre to the end of the rotor body, while another cover, half the end winding. The cooling gas for the slot portion is admitted into the hollow conductors through milled openings directly before the end of the rotor body and flows through the hollow conductors to the centre of the rotor body. The hot gas in then discharged into the air gap between the rotor body and the stator core through radial openings in the conductors and the rotor slot wedges. The cooling gas passages are arranged at different levels in the conductor assembly so that each hollow conductor has its own cooling gas outlet. The cooling gas for the end windings is admitted into the hollow conductors at the ends of the rotor body. It flows through the conductors approximately up to the pole centre for being directed into a collecting compartment and is then discharged into the air gap via slots. At the end winding, one hollow conductor passage of each bar is completely closed by a brazed copper filler section. The enlargement of the conductor rigidity. ROTOR WINDING The rotor winding consists of several coils which are reinserted into the slots and series connected such that two coil groups from one pole. Each coil consists of several series- connected turns, each of which consists of two half turns which are connected by brazing in the end section.

KORBA SIMULATOR

347

The rotor winding consists of silver bearing de-oxidised copper hollow conductors with two lateral cooling ducts. L-shaped strips of laminated epoxy glass fibre fabric with Nomex filler are used for slot insulation. The slot wedges are made of high-conductivity material and extend below the shrink seat of the retaining ring. The seat of the retaining ring is silver plated to ensure a good electrical contact between the slot wedges and rotor retaining rings. This system has long proved to be a good damper winding. The field winding are inserted into the longitudinal slots of the rotor body. The coils are wound around the poles so that one north and one south magnetic pole are obtained. The hollow conductors have a trapezoidal cross-section and are provided with two cooling ducts of approximately semi-circular cross-section. All conductors have identical copper and cooling duct cross-sections. The individual conductors are bent to obtain half turns. After insertion into the rotor slots, these turns are combined to form full turns, the series-connected turns of one slot constituting one coil. The individual coil of the rotor winding are electrically series-connected. CONDUCTOR MATERIAL The conductors are made of copper with a silver content of approximately 0.1 %. As compared to electrolytic copper, silver-alloyed copper features high strength properties at higher temperatures so that coil deformations due to thermal stresses are eliminated. INSULATION The insulation between the individual turns is made of layers of glass fibre laminate. The coils are insulated from the rotor body with L-shaped strip of glass fibre laminate with Nomex filler. To obtain the required creepage paths between the coil and the frame, thick top strips of glass fibber laminate are inserted below the slot wedges. LOCATION OF PARTS IN THE ROTOR WINDING ROTOR SLOT WEDGES To protect the winding against the effects of the centrifugal force, the winding is secured in the slots with wedges. The slot wedges are made from a copper-nickel-silicon alloy featuring high strength and good electrical conductivity, and are used as

KORBA SIMULATOR

348

damper winding bars. The slot wedges extend below the shrink seats of the retaining rings.The rings acts as short-circuit ring to induced currents in the dampers windings. END WINDING BRACING The spaces between the individual coils in the end winding are filled with insulating members, which prevent coil movement. ROTOR RETAINING RING The rotor retaining rings contain the centrifugal forces due to the end windings. One end of each ring is shrunk on the rotor body, while the other end of the ring overhangs the end windings without contacting the shaft. This ensures an unobstructed shaft deflection at the end windings. The shrunk on end ring at the free end of the retaining ring serves to reinforce the retaining ring and secures the end winding in the axial direction at the same time. A snap ring is provided for additional protection against axial displacement of the retaining ring. To reduce the stray losses and retain strength, the rings are made of non-magnetic, cold-worked material. Comprehensive tests, such as ultrasonic examination and liquid penetrate examination, ensures adherence to the specified mechanical properties. The retaining ring shrink-fit areas act as short-circuit rings to induced currents in the damper system. To ensure low contact resistance, the shrink seats of the retaining rings are coated with nickel, aluminium and silver by a three-step flame spraying process. FIELD CONNECTIONS The field connections provide the electrical connection between the rotor winding and the exciter and consists of : • Field current lead at end winding • Radial bolts • Field current lead in shaft bore

KORBA SIMULATOR

349

FIELD CURRENT LEAD AT END WINDING

The field current lead at the end winding consists of hollow rectangular conductors.

The hollow conductors are inserted into shaft slots and insulated. They are secured

against the effects of centrifugal force by steel wedges. One end of each field current

lead is brazed to the rotor winding, and the other end is screwed to a radial bolt.

Cooling hydrogen is admitted into the hollow conductors via the radial bolts. The hot

gas discharged into the air gap together with the gas used to cool the end winding. RADIAL BOLTS

The field current leads located in the shaft bore are connected to the conductors

inserted in the shaft slots through radial bolts which are secured in position with slot

wedges Contact pressure is maintained with a tension bolt and as expanding cone in

each radial bolt. Contact pressure increase due to centrifugal force during operation.

All contact surfaces are silver-plated to attain a low contact resistance. The radial bolt

is made from forged electrolytic copper.

The seal between air and hydrogen spaces is located close to the radial bolt. This seal

consists of an insulating ring which is pressed between the shaft and radial bolt with a

threaded ring.

FIELD CURRENT LEAD SHAFT BORE

The leads are run in the axial direction from the radial bolt to the exciter coupling.

They consists of two semi-circular conductors insulated from each other and from the

shaft by a tube. The field current leads are connected to the exciter leads at the

coupling with Multikontakt plug-in contacts which allow for unobstructed thermal

expansion of the field current leads.

ROTOR FAN The generator cooling gas is circulated by one axial-flow fan located on the turbine-

end shaft journal. To augment the cooling of the rotor winding, the pressure

established by the fan works in conjunction with the gas expelled from the discharge

ports along the rotor.

KORBA SIMULATOR

350

The moving blades of the fan are inserted into T shaped grooves in the fan hubs. The

fan hubs are shrink-fitted to the shaft journal spider. HYDROGEN COOLER The hydrogen cooler is a shell and tube type heat exchanger which cools the hydrogen gas in the generator. The heat removed from the hydrogen is dissipated through the cooling water. The cooling water flows through the tubes, while the hydrogen is passed around the finned tubes. The hydrogen cooler is subdivided into identical sections which are vertically mounted in the turbine-end stator end shield. The cooler sections are solidly bolted to the upper half stator end shield, while the attachment at the lower water channel permits them to move freely to allow for expansion. The cooler sections are parallel connected on their water sides. Shutoff valves are installed in the lines before and after the cooler sections. The required cooling water flow depends on the generator output and is adjusted by control valves on the hot water side. Controlling the cooling water flow on the outlet side ensures an uninterrupted water flow through the cooler sections so that proper cooler performance will not be impaired. BEARINGS The sleeve bearings are provided with hydraulic shaft lift oil during startup and turning gear operation. To eliminate shaft currents, all bearings are insulated from the stator and base plate, respectively. The temperature of the bearings is monitored with thermocouples embedded in the lower bearing sleeve so that the measuring points are located directly below the babbitt. Measurement and any required recording of the temperatures are performed in conjuction with the turbine supervision. The bearings have provisions for fitting vibration pickups to monitor bearing vibrations. SHAFT SEALS The points where the rotor shaft passes through the stator casing are provided with a radial seal ring. The seal ring is guided in the seal ring carrier which is bolted to the seal ring carrier flange and insulated to prevent the flow of shaft currents. The seal ring is lined with babbitt on the shaft journal side. The gap between the seal ring and the shaft is sealed with hydrogen side and air side seal oil. The hydrogen side seal oil

KORBA SIMULATOR

351

is supplied to the seal ring via an annular groove in the seal guide. Inside the seal ring this seal oil is fed to the hydrogen side annular groove in the seal ring and from there to the sealing gap via several bores uniformly distributed on the circumference. The air side seal oil is supplied to the sealing gap from the seal ring chamber via radial bores and the air side annular groove in the seal ring. To ensure effective sealing, the seal oil pressures in the annular gap are maintained at a higher level than the gas pressure with in the generator casing, the air side seal oil pressure being set to approximately the same level as the hydrogen side seal oil pressure. The oil drained on the hydrogen side of the seal rings is returned to the seal oil system through ducts below the bearing compartments. The oil drained on the air side is returned to the seal oil storage tank together with the bearing oil. On the air side, pressure oil is supplied laterally to the seal ring via an annular groove. This ensures free movement of the seal ring in the radial direction. OIL SUPPLY FOR BEARINGS AND SHAFT SEALS BEARING OIL SYSTEM The generator and exciter bearings are connected to the turbine lube oil supply. SEAL OIL SYSTEM Seal Oil Pump 1 & 2 Air Side

Kind of Pump : Screw Pump

Type : SNH210-R46 (Allweiler)

Capacity : 3.3 DM3/S

Discharge pressure : 15 bar

Pump motor -Type : 1LA3-133-4AA90 (Siemens)

Rating : 7.5 KW

Current : 141A

Type of enclosure : IP54

No. : 2 Nos.Full capacity

Seal Oil Pump Air Side

Kind of pump : Screw pump

Type : SNH210-R46 (All weiler)

Capacity : 3.3 DM3/S

Discharge pressure : 15bar

KORBA SIMULATOR

352

Pump motor drive : 1 HA 4165-5JL20

(Siemens)

Ranting : 8.5KW

Voltage : 220V, DC

Current : 51A

Speed : 24.17RPS

Type of enclosure : IP54

No. : 1 No. full capacity

Seal Oil Pump H2 Side

Kind of pump : Screw pump

Type : SNH 210R46 (All Weiler)

Capacity : 2.17 DM 3/S

Discharge pressure : 15 bar

Pump motor : ILA3 133-6AA90 (Siemens)

Rating : 4 KW

Current : 7.2 A

Speed : 15.8 RPS

Type of enclosure : IP54

No. : 1 No. Full capacity

Seal Oil Filter, Air Side and H2 Side

Kind of filter : Strainer Type

Type : 2.62.9 MA (BOLL+MIIRCH)

Volume flow rate : 3.3 DM3/5

Degree of filter ation : 100 Microns

No. of air side : 2 Nos. full capacity

No. of H2 side : 2 Nos. full capacity

SEAL OIL SYSTEM CONSTRUCTION The shaft seals are supplied with seal oil from two seal oil circuits which consists of the following principal components.

KORBA SIMULATOR

353

HYDROGEN SIDE SEAL OIL CIRCUIT

• Seal oil tank • Seal oil pump • Oil cooler 1 • Oil cooler 2 • Seal oil filter • Differential pressure valve C • Pressure equalising valve TE • Pressure equalising valve EE.

AIR SIDE SEAL OIL CIRCUIT

• Seal oil storage tank

• Seal oil pump 1

• Seal oil pump 2

• Standby seal oil pump

• Oil cooler 1

• Oil cooler 2

• Seal oil filter

• Differential pressure valve A1

• Differential pressure valve A2

HYDROGEN SIDE SEAL OIL CIRCUIT The seal oil drained towards the hydrogen side is collected in the seal oil tank. The associated seal oil pump returns the oil to the shaft seals via a cooler and filter. The hydrogen side seal oil pressure required downstream, of the pump is controlled by differential pressure valve C according to the preset reference value, i.e. the preset difference between air side and hydrogen side seal oil pressures. The hydrogen side seal oil pressure required at the seals is controlled separately for each shaft seal by the Exciter end or Turbine end pressure equalising valve, according to the preset pressure difference between the hydrogen side and air side seal oil. Oil drained from the hydrogen side is returned to the seal oil tank via the generator prechambers. Two float operated valves keep the oil level at a predetermined level, thus preventing gas from entering the suction pipe of the seal oil pump (hydrogen side). The low level float operated valve compensates for an insufficient oil level in the tank by admitting oil from the air side seal oil circuit. The high level float operated valve drains excess oil into the seal oil storage tank. The hydrogen entrained in the

KORBA SIMULATOR

354

KORBA SIMULATOR

355

seal oil comes out of the oil and is extracted by the bearing vapour exhauster for being vented to the atmosphere above the power house roof. During normal operation, the high level float-operated drain valve is usually open to return the excess air side seal oil, which flowed to the hydrogen side via the annular gaps of the shaft seals, to the air side seal oil circuit. Air Side Seal Oil Circuit The air side seal oil is drawn from the seal oil storage tank and delivered to the seals via a cooler and filter by seal oil pump 1. In the event of a failure of seal oil pump 1 of the air side seal oil circuit, seal oil pump 2 automatically takes over the seal oil supply. Upon failure of seal oil pump 2, the standby seal oil pump is automatically started and takes over the seal oil supply to the shaft seals. In the event of a failure of the seal oil pump of the hydrogen side seal oil circuit, the seal oil is taken from the air side seal oil circuit. The air side seal oil pressure required at the seals is controlled by differential pressure valve A1 according to the preset value, i.e. the required pressure difference between seal oil pressure and hydrogen pressure. In the event of a failure, i.e. when the seal oil for the seals is obtained from the standby seal oil pump, differential pressure valve A2 takes over this automatic control function. The seal oil drained from the air side of the shaft seals is directly returned to the seal oil storage tank. GAS SYSTEM General The gas system contains all equipment necessary for filling the generator with CO2,

hydrogen or air and removal of these media, and for operation of the generator filled with hydrogen. In addition, the gas system includes a nitrogen (N2) supply. The gas system consists of :

• H2 supply

• CO2 supply

• N2 supply

• Pressure reducers

• Pressure gauges • Miscellaneous shut off valves

• Purity metering equipment • Gas dryer • CO2 flash evaporator

• Flowmeters.

KORBA SIMULATOR

356

HYDROGEN GAS SYSTEM

HYDROGEN (H2 ) SUPPLY

GENERATOR CASING

The heat losses arising in the generator are dissipated through hydrogen. The heat

dissipating capacity of hydrogen is eight times higher than that of air. For more effective cooling, the hydrogen in the generator is pressurized.

KORBA SIMULATOR

357

PRIMARY WATER TANK Nitrogen environment is maintained above the primary water in the primary water

tank:

• To prevent the formation of a vacuum due to different thermal expansions of the primary water.

• To ensure that the primary water in the pump suction line is at a pressure above atmospheric pressure so as to avoid pump cavitation.

• To ensure that the primary water circuit is at a pressure above atmospheric

pressure so as to avoid the ingress of air on occurrence of a leak. CARBON DIOXIDE (CO2 ) SUPPLY

As a precaution against explosive hydrogen air mixtures, the generator must be filled with an inert gas (CO2 ) prior to H2 filling and H2 removal.

The generator must be filled with (CO2 ) until it is positively ensured that no

explosive mixture will form during the subsequent filling or emptying procedures. COMPRESSED AIR SUPPLY To remove the CO2 from the generator, compressed air must be admitted into the

generator. The compressed air must be clean and dry. For this reason, a compressed air filter is installed in the filler line. NITROGEN (N2 ) SUPPLY

Nitrogen is required for removing the hydrogen or air during primary water filling and emptying procedures. PRIMARY WATER SYSTEM GENERAL The primary water required for cooling is circulated in a closed circuit by a separate pump. To ensure uninterrupted generator operation, two full-capacity pumps are provided. In the event of a failure of one pump, the standby pump is immediately ready for service and cuts in automatically. Each pump is driven by a separate motor.

KORBA SIMULATOR

358

KORBA SIMULATOR

359

All valves, pipes and instruments coming into contact with the primary water are made from stainless material. The primary water system consists of the following principal components:

• Primary water tank • Primary water pumps • Primary water coolers • Fine filter • Ion Exchanger.

As illustrated in the diagram, the primary water admitted to the pump from the tank is first passed via the cooler and fine filter to the water manifold in the generator interior and then to the bushings. After having performed its cooling function, the water is returned to the primary water tank. The gas pressure above the water level in the primary water tank is maintained constant by a pressure regulator. PRIMARY WATER TANK The primary water tank is located on top of the stator frame on an elastic support, thus forming the highest point of the entire primary water circuit in terms of static head. PRIMARY WATER TREATMENT SYSTEM The direct contact between the primary water and the high-voltage windings call for a low conductivity of the primary water. During operation, the electrical conductivity should be maintained below a value of approximately 1 mmho/cm. In order to maintain such a low conductivity it is necessary to provide for continuous water treatment during operation, a small quantity of the primary water should therefore be continuously passed through the ion exchanger located in the bypass of the main cooling circuit. The ion exchanger resin material requires replacement at infrequent intervals. The resins can be replaced during operation of the generator, since with the water treatment system out of service, the conductivity will rise very slowly. STATOR To facilitate manufacture, erection and transport, the stator consists of the following main components :

• Stator frame

KORBA SIMULATOR

360

• End shields • Bushing compartment

The stator frame with flexible core suspension components, core, and stator winding is the heaviest component of the entire generator. A rigid frame is required due to the forces and torque’s arising during operation. In addition, the use of hydrogen for the generator cooling requires the frame to be pressure resistant up to an internal pressure of approximately 10 bar (130 psig). The welded stator frame consists of the cylindrical frame housing, two flanged rings and axial and radial ribs. Housing and ribs within the range of the phase connectors of the stator winding are made of non-magnetic steel to prevent eddy current losses, while the remaining frame parts are fabricated from structural steel. The arrangement and dimensioning of the rib are determined by the cooling gas passages and the required mechanical strength and stiffness. Dimensioning is also dictated by vibrational considerations, resulting partly in greater wall thickness than required from the point of view of mechanical strength. The natural frequency of the frame does not correspond to any exciting frequencies. Two lateral supports for flexible core suspension in the frame are located directly adjacent to the points where the frame is supported on the foundation. Due to the rigid design of the supports and foot portion the forces due to weight and shot-circuit will not result in any over-stressing of the frame. Manifolds are arranged inside the stator frame at the bottom and top for filling the generator with CO2 and H2. The connections of the manifolds are located side by side in the lower part of the frame housing. Additional openings in the housing, which are sealed gas tight by pressure-resistant covers, afford access to the core clamping flanges of the flexible core suspension system and permit the lower portion of the core to be inspected. Access to the end winding compartments is possible through manholes in the end shields. In the lower part of the frame at the exciter end an opening is provided for bringing out the winding ends. The generator terminal box is flanged to this opening. STATOR END SHIELDS The ends of the stator frame are closed by pressure containing end shields. The end shields feature a high stiffness and accommodate the generator bearings, shaft seals and hydrogen coolers. The end shields are horizontally split to allow for assembly. The end shields contain generator bearings. This results in a minimum distance between bearings and permits the overall axial length of the Turbine end shield to be utilised for accommodation of the hydrogen cooler sections. Cooler wells are provided

KORBA SIMULATOR

361

in the end shield on both sides of the bearing compartment for this purpose. One manhole in both the upper and lower half end shield provides access to the end winding compartments of the completely assembled machine. Inside the bearing compartment the bearing saddle is mounted and insulated from the lower end shield. The bearing saddle supports the spherical bearing sleeve and insulates it from ground to prevent the flow of shaft currents. The bearing oil is supplied to the bearing saddle via pipe permanently installed in the end shield and is then passed on to the lubricating gap via ducts in the lower bearing sleeve. The bearing drain oil is collected in the bearing compartment and discharged from the lower half of the end shield via a pipe. The bearing compartment is sealed on the air side with labyrinth rings. On the hydrogen side the bearing compartment is closed by the shaft seal and labyrinth rings. The oil for the shaft seal is admitted via integrally welded pipes. The seal oil drained towards the air side is drained together with the bearing oil. The seal oil drained towards the hydrogen side is first collected in a gas and oiltight chamber below the bearing compartment for defoaming and then passed via a siphon to the seal oil tank of the hydrogen side seal oil circuit. The static and dynamic bearing forces are directly transmitted to the foundation via lateral feet attached to the lower half end shield. The feet can be detached from the end shield, since the end shields must be lowered into the foundation opening for rotor insertion. GENERATOR TERMINAL BOX The phase and neutral leads of the three-phase stator winding are brought out of the generator through six bushings located in the generator terminal box at the exciter end of the generator. The terminal box is a welded construction of non-magnetic steel plate. This material reduces stray losses due to eddy currents. Welded ribs provide for the rigidity of the terminal box. Six manholes in the terminal box provide access to the bushing during assembly and overhauling.

KORBA SIMULATOR

362

STATOR CORE In order to minimise the hysteresis and eddy current losses of the rotating magnetic flux which interacts with the core, the entire core is built up of thin laminations. Each lamination layer is made up from a number of individual segments. The segments are punched in one operation from 0.5 mm (0.02 in.) thick electrical sheet-steel laminations having a high silicon content, carefully deburred and then coated with insulating varnish on both sides. The stator frame is turned on end while the core is stacked with lamination segments in individual layers. The segments are staggered from layer so that a core of high mechanical strength and uniform permeability to magnetic flux is obtained. On the outer circumstance the segments are stacked on isolated dovetail bars which hold them in position. One dovetail bar is not insulated to provide for grounding of the laminated core. Stacking guides inserted into the winding slots during stacking provide smooth slot walls. To obtain the maximum compression and eliminate undue settling during operation, the laminations are hydraulically compressed and heated during the stacking procedure when certain heights of stack are reached. The complete stack is kept under pressure and located in the frame by means of clamping bolts and pressure plates. The clamping bolts running through the core are made of non-magnetic steel and are insulated from the core and the pressure plates to prevent the clamping bolts from short-circuiting the laminations and allowing the flow of eddy currents. The pressure is transmitted from the pressure plates to the core by clamping fingers. The clamping fingers extend up to the ends of the teeth thus ensuring a firm compression in the area of the teeth. The stepped arrangement of the laminations at the core ends provides for an efficient support of the tooth portion and in addition, contributes to a reduction of eddy current losses and local heating in this area. The clamping fingers are made of non-magnetic steel to avoid eddy current losses. For protection against the effects of the stray flux in the coil ends, the pressure plates and core end portions are shielded by gas-cooled rings of insulation-bonded electrical sheet-steel. To remove the heat, spacer segments, placed at intervals along the bore length, divide the core into sections to provide radial passages for cooling gas flow. In the core end portions, the cooling ducts are wider and spaced more closely to account for the higher losses and to ensure more intensive cooling of the narrow core sections. SPRING SUPPORT OF STATOR CORE The revolving magnetic field exerts a pull on the core, resulting in a revolving and nearly elliptical deformation of the core which sets up a stator vibration at twice the system frequency. To reduce the transmission of these dynamic vibrations to the

KORBA SIMULATOR

363

foundation, the generator core is spring mounted in the stator frame. The core is supported in several sets of rings. Each ring set consists of two supporting rings and two core clamping rings. The structural members to which the insulated dovetail bars are bolted are uniformly positioned around the supporting ring interior to support the core and to take up the torque acting on the core. For firm coupling of the ring sets to the core, the supporting ring is solidly pressed against the core by the clamping ring. The clamping ring consists of two parts which are held together by two clamps. Tightening the clamps reduces the gap between the ring segments so that the supporting ring is pressed firmly against the core. Each ring set is linked to the frame by three flat springs. The core is supported in the frame via two vertical springs in the vicinity of the generator feed. The lower spring prevents a lateral deflection of the core. The flat springs are resilient to radial movements of the core suspension points and will largely resist transmission of double frequency vibration to the frame. In the tangential direction they are however, sufficiently rigid to take up the short-circuit torque of the unit. The entire vibration system is turned so as to avoid resonance with vibrations at system frequency or twice the system frequency. STATOR WINDING GENERAL, CONNECTION The three-phase stator winding is a fractional-pitch two-layer type consisting of individual bars. Each stator slot accommodates two bars. The slot bottom and top bars are displaced from each other by one winding pitch and connected at their ends to form coil groups. The coil group are connected together with phase connectors inside the stator frame. This arrangement and the shape of the bars at the ends result in a cone shaped winding having particularly favourable characteristics both in respect of its electrical properties and resistance to magnetically induced forces. The bars afford maximum operating reliability, since each coil consists of only one turn. This makes the turn insulation and themaaiiin insulation identical. CONDUCTOR CONSTRUCTION The bar consists of a large number of separately insulated strands which are transposed to reduce the skin effect losses.

KORBA SIMULATOR

364

The strands of small rectangular cross-section are provided with a braided glass fibber insulation and arranged side by side over the slot width. The individual layers are insulated from each other by a vertical separator. In the straightslot portion the strands are transposed by 540 deg. The transposition provides for a mutual neutralisation of the voltages induced in the individual strands due to the slot cross field and end winding flux leakage and ensures that minimum circulating currents exist. The current exist. The current flowing through the conductor is thus uniformly distributed over the entire bar cross-section so that the current-dependent losses will be reduced. The alternate arrangement of one hollow strand and two solid strands ensures optimum heat removal capacity and minimum losses. At the Roebel crossover points, the insulation is reinforced with insulating strip inserts. To ensure that the strands are firmly bonded together and to give dimensional stability in the slot portion, the bars are cured in an electrically heated press. Prior to apply the bar insulation, the bar ends are bent with a special device which shapes the involutes over a cone shell. This ensures a uniform spacing of the bars over the entire length of the end turns after installation. Contact sleeves for electrical connection of the bars and water boxes with the cooling water connections are brazed to the bar ends. In the course of manufacture, the bars are subjected to numerous electrical and leakage tests for quality control. CORONA PROTECTION To prevent potential differences and possible corona discharges between the insulation and the slot wall, the slot sections of the bars are provided with an outer corona protection. This protection consists of a wear-resistant, highly flexible coating of conductive alkyd varnish containing graphite. At transition from the slot to the end winding portion of the stator bars, a semi-conductive coating is applied. On top of this, several layers of semi-conductive end corona protection coating are applied in varying lengths. This ensures uniform control of the electric field and prevents the formation of corona discharge during operation and during performance of high voltage tests. A final wrapping of glass fabric tapes impregnated with epoxy resin serves as surface protection.

KORBA SIMULATOR

365

COMPONENTS FOR WATER COOLING OF STATOR WINDINGS GENERAL Two separate water cooling circuits are used for the stator winding and the phase connectors and bushings. All water connection between ungrounded parts and the distribution manifolds & water manifolds of the cooling circuits are insulated teflon hoses. The water connections are equipped with O-rings of Viton and Belleville washers to prevent loosening of the connection. The fittings are made from non-magnetic stainless steel. WINDING COOLING CIRCUIT The end windings are enclosed by an annular water manifold to which all stators bars are connected through hoses. The water manifold is mounted on the holding plates of the end winding support ring and connected to the primary water supply pipe. This permits the insulation resistance of the water-filled stator winding to be measured. The water manifold is grounded during operation. For measurement of the insulation resistance, e.g. during inspections, grounding is removed by opening the circuit outside the stator frame. The hoses, one side of which is connected to ground, consists of a metallic section to which the measuring potential is applied for measurement of the insulation resistance of the water-filled stator winding. The cooling water is admitted to three terminal bushings via a distribution water manifold, flows through the attached phase connectors and is then passed to the distribution water manifold for water outlet via the terminal bushings on the opposite side. The parallel-connected cooling circuits are checked for uniform water flows by a flow measurement system covering all three phases. The cooling primary water flows through the stator bars, which are hydraulically connected in parallel, from the exciter end to the turbine end of the generator. This ensures a minimum temperature rise of the stator bars, a minimum water velocity, and a minimum head loss. Moreover, the thermal expansions of the stator bars are completely uniform.

KORBA SIMULATOR

366

PHASE CONNECTOR COOLING CIRCUIT Phase connectors and terminal bushings are supplied with cooling water through pipes arranged outside the generator at the terminal bushing and generator terminal box and connected to the cooling water inlets and outlets of the cooling circuit through Teflon hoses. The flexible expansion joints and the hydraulically series-connected phase connector sections are connected by Teflon hoses. The hoses, one side of which is connected to ground, consists of a metallic section to which the measuring potential is applied for measurement of the insulation resistance of the water-filled stator winding. The cooling water is admitted to three terminal bushings via a distribution water manifold, flows through the attached phase connectors and is then passed to the distribution water manifold for water outlet via the terminal bushings on the opposite side. The parallel-connected cooling circuits are checked for uniform water flows by a flow measurement system covering all three phases. EXCITATION SYSTEM 500 MW INTRODUCTION In 500 MW Turbo-generator, brushless excitation system is provided. Brushless exciter consists of a 3 phase permanent magnet pilot exciter the output of which is rectified and controlled by the Thyristor Voltage Regulator to provide a variable d.c. current for the main exciter. The 3 phases are induced in the rotor of the main exciter and is rectified by the rotating diodes and fed to the field winding of generator rotor through the D.C. leads in the rotor shaft. Since the rotating rectifier bridge is mounted on the rotor, the slip rings are not required and the output of the rectifier is connected directly to the field winding through the generator rotor shaft. A common shaft carries the rectifier wheels, the rotor of the main exciter and permanent magnet rotor of the pilot exciter. The voltage regulation is effected by using thyrism 04.2, an automatic voltage regulator. There are two independent control systems right up to the final Thyristor element-an auto control and a manual control. The control is effected on the 3 phase output of the pilot exciter and provides a variable d.c. input to the main exciter. The feedback of voltage and current output of the generator is fed to the AVR where it is compared with the set-point generator volts set from the control room. The current feedback is utilised for active and reactive power compensation and for the limiters.

KORBA SIMULATOR

367

KORBA SIMULATOR

368

There are 3 limiters, under excitation limiter, over excitation limiter and U/Hz limiter which act on the AVR. A power system stabiliser is also envisaged for damping oscillations in the power system. The manual control system consists of an excitation controller which control the excitation as set on the manual set-point from the control room. A field forcing limiter allows field forcing during emergency upto the capability of the main exciter. In cases of defects in the automatic control system the excitation automatically changes over the manual regulation through protective relays. In order to ensure a bumpless transfer follow up circuit controls the manual channel so that it follows the auto channel continuously. De-excitation of the machine is effected by driving the thyristors to inverter mode of operation causing the thyrister to supply maximum reverse voltage to the field winding of the main exciter. Approximately 0.5 secs. after de-excitation command is received two field suppressions contractors connect field suppression resistors in parallel to main exciter field winding and following this a trip command is transmitted to the field circuit breaker via its trip coil. In the event of a failure of the electronic de-excitation through inverter operation, de-excitation would be effected with a delay of 0.5 seconds by field suppression resistors. The main advantage of rotating diode excitation system is that it eliminates the use of slip rings and carbon brushes which pose constant maintenance problems. Following chapters deal with the design features, constructional details and basic operation of the excitation system. The first part will deal with the basic design features and will illustrate the Basic Arrangement of Brushless Excitation System with rotating diodes along with the constructional details of the system. The second part will describe the voltage regulator, its Basic mode of operation along with the limiters. The three-phase pilot exciter has a revolving field with permanent magnet poles. The three-phase ac generated by the permanent magnet exciter is rectified and contributed by the TVR to provide a variable de current for exciting the main exciter. The three phase AC induced in the rotor of the main exciter is rectified by the rotating rectifier bridge and led to the field winding of the generator rotor through the DC leads in the rotor shaft. A common shaft carries the rectifier wheels, the rotor of the main exciter and the permanent magnet rotor of the pilot exciter. The shaft is rigidly coupled to the

KORBA SIMULATOR

369

generator rotor. The exciter shaft is supported on a bearing between the main and pilot exciters. The generator and exciter rotors are thus supported on total of three bearings. Mechanical coupling of the two shaft assemblies results in simultaneous coupling of the dc leads in the central shaft bore through the Multikontakt electrical contact system consisting of plug-in bolts and sockets. This contact system is also designed to compensate for length variations of the leads due to thermal expansion. RECTIFIER WHEELS The main components of the rectifier wheels are the silicon diodes which are arranged in the rectifier wheels in a three phase bridge circuit. The contact pressure for the silicon wafer is produced by a plate spring assembly. The arrangement of the diode is such that this contact pressure is increased by the centrifugal force during rotation.

Two diodes each are mounted in each aluminium alloy heat sink and thus connected

in parallel. Associated with each heat sink is a fuse which serves to switch off the two

diodes if one diodes fails (loss or reverse blocking capability). Following are the basic

elements of rectifier wheels:

1. Rectifier wheel

2. Three phase lead

3. Heat sink

4. Diode

5. Fuse

6. Multikontakt plug -in-bolt For suppression of the momentary voltage peaks arising from commutation, each wheel is provided with six RC networks consisting of one capacitor and one damping resistor each which are combined in a single resin-encapsulated unit. The insulated and shrunken rectifier wheels serves as DC buses for the negative and positive side of the rectifier bridge. This arrangement ensures good accessibility to all components and a minimum of circuit connections. The two wheels are identical in their mechanical design and differ only in the forward directions of the diodes. The direct current from the rectifier wheels is fed to the dc leads arranged in the centre bore of the shaft via radial bolts. The three-phase alternating current is obtained via copper conductors arranged on the shaft circumference between the rectifier wheels and the three-phase main exciter. The conductors are attached by means of banding clips and equipped with screw-on

KORBA SIMULATOR

370

lugs for the internal diode connections. One three-phase conductor each is provided for the four diodes of a heat sink set. THREE-PHASE MAIN EXCITER MAIN COMPONENTS OF 3 PHASE EXCITER ARE :

1. Rotor 2. Stator 3. Magnetic pole 4. Sliprings for ground fault detection 5. Bearing housing

The three-phase main exciter is a six-pole revolving-armature unit. Arranged in the stator frame are the poles with the field and damper winding. The field winding is arranged on the laminated magnetic poles. At the pole shoe bars are provided their ends being connected so as to form a damper winding. Between two poles a quadrature-axis coil is fitted for inductive measurement of the exciter current. The rotor consists of stacked laminations which are compressed by through bolts over compression rings. The three-phase winding is inserted in the slots of the laminated rotor. The winding conductors are transposed within the core length, and the end turns of the rotor winding are secured with steel bands. The connections are made on the side facing the rectifier wheels. The winding ends are run to a bus ring system to which the three-phase leads to the rectifier wheels are also connected. After full impregnation with synthetic resin and curing, the complete rotor is shrunk on to the shaft. A journal bearing is arranged between main exciter and pilot exciter and has forced oil lubrication from the turbine oil supply. THREE-PHASE PILOT EXCITER The three-phase pilot exciter is a 16 pole revolving-field unit. The frame accommodates the laminated core with the three-phase winding. The rotor consists of a hub with mounted poles. Each pole consists of 10 separate permanent magnets which are housed in a non-magnetic metallic enclosure. The magnets are braced between the hub and the external pole shoe with bolts. The rotor hub is shrunk onto the free shaft end. COOLING OF EXCITER The exciter is air cooled. The cooling air is circulated in a closed circuit and recooled in two cooler sections arranged along side the exciter.

KORBA SIMULATOR

371

The complete exciter is housed in an enclosure through which the cooling air circulates.The rectifier wheels, housed in their own enclosure draw the cool air in at both ends and expel the warmed air to the compartment beneath the base plate. The main exciter enclosure receives cool air from the fan after it passes over the pilot exciter. The air enters the main exciter from both ends and is passed into ducts below the rotor body and discharged through radial slots in the rotor core to the lower compartment. The warm air is then returned to the main enclosure via the cooler sections.

EMERGENCY COOLING OF EXCITER

Emergency cooling is provided to permit continued operation in the event of cooler

failure. In such an emergency, flaps in the hot and cold air compartments are

automatically operated by actuators admitting cold air from outside the exciter

enclosure and discharge the hot air through openings in the base frame. Main parts of

permanent Magnet pilot exciter are:

1. Stator 2. Permanent-magnet rotor 3. Stator winding

REPLACEMENT OF AIR INSIDE EXCITER ENCLOSURE When the generator is filled with hydrogen (operation or standstill) an adequate replacement of the air inside the exciter enclosure must be ensured. The air volume inside the exciter enclosure requires an air change rate of 125 m3/hr.While the generator is running the air leaving the exciter enclosure via the bearing vapour exhaust system and the leakage air outlet in the foundation provides for a pull-through system. The volume of air extracted from the cooling air circuit is replaced via the filters located at the top of the enclosure: When the generator is at rest the air dryer of the exciter unit discharges dry air inside the exciter enclosure. The air leaves the exciter enclosure via the leakage air filter and the leakage air outlet at the shaft as well as via the bearing vapour exhaust system if this system is in service. EXCITER DRYING GENERAL A dryer (dehumidifier) and an anticondensation heating system are provided to avoid the formation of moisture condensate inside the exciter with the turbine generator at rest or on turning gear.

KORBA SIMULATOR

372

MODE OF OPERATION The dryer dehumidifies the air within the exciter enclosure. The dryer wheel is made of a non-flammable material. On its inlet side, the wheel is provided with a system of tubular ducts, the surfaces of which are impregnated with a highly hygroscopic material. The tubular ducts are dimensioned so that a laminar flow with low pressure loss is obtained even at high air velocity. The moisture absorbed by the dryer wheel is removed in a regeneration section by a stream of hot air directed through the wheel in the opposite direction of the inlet air and then discharged to the atmosphere. After regeneration, the dryer wheel material is again capable of absorbing moisture. The adsorption of moisture and regeneration of the dryer wheel material take place simultaneously, using separate air streams, which ensures a continuous drying of the air. A shutoff valve in the dry air outlet line prevents that contaminated air from the power house which will be drawn during load operation of the exciter: OPERATING PRINCIPLE OF ADSORPTION DRYER The dehumidification takes place in a slowly rotating dryer wheel (approximately 7 revolutions per hour). The honeycomb dryer wheel consists of a magnesium silica alloy containing crystalline lithium chloride. The inlet side of the dryer wheel is subdivided so that 1/4 is available for regeneration and 3/4 for the adsorption section. ADSORPTION SECTION The air to be dehumidified passes through the absorption section of the dryer wheel, with part of the moisture contained in the air being removed by the adsorbent material, i.e. lithium chloride. The moisture is removed as a result of the partial pressure drop existing between the air and the adsorbent material. REGENERATION SECTION In the regeneration section of the dryer wheel, the accumulated moisture is removed from the dryer wheel by the heated regeneration air.

KORBA SIMULATOR

373

Continuous rotation of the dryer wheel ensures continuous dehumidification of the air within the exciter. ANTICONDENSATION HEATING SYSTEM An anticondensation heating system to support the dryer is installed in the exciter baseframe. The heaters are rated and arranged so that the temperature in the exciter interior is maintained above the dew point level. The heaters are controlled through rod-type thermostats located in the exciter interior. GROUND FAULT DETECTION SYSTEM The field ground fault detection system detects high resistance and low-resistance ground faults in the exciter field circuit. This is very important for safe operation of a generator, because a double fault causes magnetic unbalances, with very high currents flowing through the faulted part, resulting in its destruction within a very short time. It is therefore an essential requirement that even simple ground faults should activate an alarm and protective measures be initiated, if possible, before the fault can fully develop. For this reason, the field ground fault detection system consists of two stages and operates continuously. If the field ground fault detection system detects a ground fault, an alarm is activated at . If the insulation resistance between the exciter field circuit and ground either suddenly or slowly drops to the generator electrical protection is tripped (2nd stage). The generator is thus automatically disconnected from the system and de-excited. AUTOMATIC VOLTAGE REGULATOR VOLTAGE REGULATING SYSTEM Type : Thyrisiem 04-2

Maximum output voltage : 250V

Output current for field forcing : 152A

Output current for rated generator load : 88A

Auxiliary voltage from pilot exciter for thyristor

sets

: Three phase supply

220 V,400 Hz

D.C.voltage from station battery for conductor &

drives

: 220V

Power input continuously : < 0.1KW

Power input short time : < 1KW

KORBA SIMULATOR

374

DC current from station battery 2 X 24 V for

control and regulation

: Max. 15A positive

Max.6A Negative

Rated secondary voltage : 120V

Power input of voltage transformer per phase : 2 VA

Rated secondary current : 5A

Power input of current transformer per phase : 6.5 VA (plus losses in

connecting leads)

Accuracy of control : better than ± 0.5%

Setting range of voltage set point potentiometer : + 5-10%of nominal Gen. voltage

Setting range of droop compensation or

compounding

: ± 0-10% dependent on the

setting of the potentiometer and

proportional to reactive current

BASIC MODE OF OPERATION The THYRISIEM 04-2 voltage regulator is designed for excitation and control brushless generators. The block diagram shows the circuit configuration. The machine set consists of the generator and a direct coupled exciter unit with a three phase main exciter, rotating rectifiers and a permanent magnet auxiliary exciter. The main components of the voltage regulator are two closed-loop control systems each followed by a separate gate control unit and Thyristor set and a de-excitation equipment. In addition to this (but not shown), a open-loop control system for the signal exchange between the regulator and the power station control room and other plant components is provided as well as power supply equipment. Control system 1 for automatic generator voltage control (AUTO) comprises the following :

• Generator voltage control; the output quantity of this control is the set-point for a following

• Excitation current regulator, controlling the field current of the main exciter (=

output current of the co-ordinated Thyristor set)

• Circuit for automatic excitation build-up during start-up and field suppression during shut-down; this equipment acts onto the output of the generator voltage control, limiting the set-point for the above excitation current regulator. The stationary value of this limitation determines the maximum possible excitation current set-point (field forcing limitation);

• Limiter for the under-excited range (under excitation limiter),

KORBA SIMULATOR

375

• Delayed limiter for the over excited range (over excitation limiter).

The field forcing limitation limits - practically undelayed - the output current of the thyristor sets to the maximum permissible value, when the voltage regulation calls for maximum excitation. Normally, this maximum permissible value is 1.5 times the rated excitation. The over excitation limiter ensures delayed reduction of the excitation current to the rated value in the over excited range, i.e. between rated excitation and maximum excitation. The delay time depends on the amount by which the rated value has been exceeded. These limiters protect Thyristor sets and machines against over excitation with too high values or too long duration. In the under-excited range, the under excitation limiter ensures that the minimum excitation required for stable parallel operation of the generator with the system is available and that the under-excited reactive power limited accordingly. The response characteristic is formed on the basis of the generator reactive current, active current and terminal voltage and can be matched to the generator and system data. Control system 2 (MANUAL) mainly comprises a second excitation current regulator with separate sensing for the actual value. This control system is also called Manual control system, because for constant generator voltage manual re-adjusting of the excitation current set-point is required when changing the generator load. The excitation current regulator permits plotting of generator characteristics and setting of protective relays during no-load and short-circuit runs of the generator during commissioning and maintenance work. The system can also be used for setting the generator excitation during normal operation when the automatic voltage is defective. Normally, the automatic voltage regulator is in service even during start-up and shut down of the generator set. The set-point adjuster of the excitation current regulator for MANUAL is tracked automatically (follow-up control) so that, in the event of faults, changeover to the MANUAL control system is possible without delay. Automatic changeover to the MANUAL control system is possible without delay. Automatic changeover is initiated by some special fault conditions. Correct operation of the follow-up control circuit is monitored and can be observed on a matching instrument in the control room. This instrument can also be used for manual matching. Either control system is co-ordinated with a separate gate-control and Thyristor set. Separate equipment is also provided for supplying power to either control system. The two separate Thyristor sets for automatic voltage regulation (AUTO) and excitation current control (MANUAL) have the same ample dimensioning regarding rated current and blocking voltage. Each Thyristor is fused separately. The Thyristor set for automatic voltage regulation can be switched off by means of an isolator with contacts in the gate-control, power supply and output sides.

KORBA SIMULATOR

376

KORBA SIMULATOR

377

This isolator in conjunction with corresponding arrangement and design of the Thyristor set enables an exchange of thyristors and fuses during operation if necessary whilst operation is continued by means of the excitation current regulator (MANUAL). In addition, the Thyristor set for automatic voltage regulation is equipped with a current-flow monitoring system for detecting failure of firing pulses or fuses. Automatic changeover to the current regulator (MANUAL) is initiated by this system. On the input side, the Thyristor sets are fed with auxiliary power from a 220V, 400 Hz. Permanent magnet auxiliary exciter. The output side of the Thyristor sets feeds the field winding of the main exciter with variable D.C. current. To de-excite the generator during shutdown or when the generator protection system has picked up, a command is transmitted to the outputs of both control systems, driving the Thyristor set being in service to maximum negative output voltage. The negative voltage (inverter operation) de-excites the main exciter in less than 1/2 sec. The generator de-excitation following is a function of the relevant effective generator time constant. Approximately 1/2 sec. after receiving the de-excite command, two field suppression contractors (one being redundant) switch a field discharge resistor in parallel to the main exciter field winding. Subsequently an off command is issued to the field breaker via its tripping coil. In the event of failure of the electronic field suppression by inverter operation, de-excitation would be achieved with a delay of 1/2 sec.via the field discharge resistors. The THYRISIEM 04-2 voltage regulator equipment is arranged within the cubicle group selected according to the power circuits and the 24 V D.C. or 15 V D.C. open and closed-loop control circuits. The signal exchange between the power circuits and the electronic circuits is via voltage isolating transducers, transformers and coupling relays. The closed-loop control systems are made up of modules of the simadyn C system whereas modules of the simatic c1 system are used for the electronic open-loop control and the alarm system. CONNECTION Closing of the field breaker from the control room or from a functional group equipment is controlled by an Iscamatic control module AS11. Off (de-excite) commands are issued from the generator protection and from the emergency push-button (via the generator protection) at the 220 V level. When the generator is being shut down, speed criteria cause the field circuit breaker to be tripped (de-excitation). The Off pushbutton in the control room normally is only provided, to reset the Iscamatic control module in these cases. The voltage regulator issues the checkback signals “Field breaker Off/On” and “Control voltage fault” to the control room. The latter is issued if one or both of the trip voltages for the field breaker are faulty. The pushbutton “MATCHING” which is used for manual matching during AUTO-MANUAL changeover and the associated signal lamps. Only one set of pushbuttons

KORBA SIMULATOR

378

LOWER/RAISE is provided for the control of the set-point adjusters. The commands go automatically to the set-point adjuster depending on the mode of operation selected, also during matching. The position of both set-point adjusters is qualitatively indicated by two instruments. The instrument “MATCH” is necessary for manual matching. It also enables the automatic follow-up to be checked. During steady-state operation, the matching instrument must indicate approximately zero. LOWER/RAISE command also come from the synchronisation unit. A checkback signal “Generator voltage > 90%’’ is formed in the regulator for used as a enabling criterion, available for a functional group control. The alarm “AVR fault” is a group alarm; the triggering individual alarms appear on indicator modules in the regulator cubicle. The alarm “Autom. Changeover to MANUAL” is issued if due to a fault criterion within the regulator automatic changeover to excitation current control (MANUAL) takes place. The alarms Excitation Low/High appear when the under - or over-excitation limiter is in action. The input “AUTO command” and checkback signal “AUTO” are required to set the regulator to the AUTO-mode by the functional group control equipment prior to automatically starting up turbo-set. A twin supply from the 220 V battery. This is used for supplying the field breaker motor drive and the two field breaker trip loops. The power supply inputs of both trip loops are wired to terminals; this offers the possibility, in case of twin channel generator protection one trip loop each to be assigned to the power supply and the trip command of the two protection channels. The 24 V power supply for open-loop and closed-loop control circuits is also a twin supply. Short-circuit protection of the voltage transformers is ensured by an MCB connected to the secondaries. Tripping of this MCB initiates automatic changeover to MANUAL. The 220 V, 400 Hz auxiliary power for the Thyristor sets is fed to the regulator via a power cable. The voltage of the auxiliary exciter is largely proportional to the speed and is used as speed criterion in the voltage regulator. To eliminate the load current dependent voltage drop on the cable from the measured value in case of large distances between the machine set and the regulator cubicle, an unloaded cable for the 400 Hz measuring are fused on the machine side and monitored for undervoltage on the regulator side. The output voltage of the Thyristor sets is available for measuring purpose (e.g. for the under-excitation protection system) at terminals protected by low-rated MCB’s.

KORBA SIMULATOR

379

In addition to the speed criterion derived from the auxiliary exciter voltage, the speed value n < 2790 rpm is provided from a speed limit monitor as a redundant criterion. The speed criteria are used for enabling the excitation during start-up and for automatic de-excitation of the generator during shutdown. The criteria Generator loaded/not loaded are required for enabling or inter-locking some control and monitoring functions. If applicable, a breaker to feed the station supply bus from the generator terminals is to be taken into account accordingly EXCITATION CONTROL DURING START-UP AND SHUTDOWN, FIELD BREAKER CONTROL, DE-EXCITATION Excitation and voltage closed-loop control are not necessary for speeds under approx. 0.95 times rated speed. Furthermore closed-loop control of the generator voltage to the rated voltage would not be permissible at low speeds since the generator and unit transformer would become saturated. For this reason, functions are provided for enabling excitation during start-up and for blocking excitation during shutdown of the generator. The speed is detected via the largely speed-proportional voltage of the auxiliary exciter. In addition to this, redundant speed criteria n< and n> are used from a speed limit monitor, if available. The field breaker is to be switched on after reaching the speed limit required by a manual command or from a functional group control system. In both cases the command passes, as well as the check-back signals “Field breaker Off/On” and “Control voltage fault” through a Iscamatic control module AS11; Closing of the field breaker is interlocked with the criterion “Ramp function generator lower limit” to ensure that the generator voltage builds up slowly without overshooting. During excitation current control (MANUAL), the lower limit of the set-point adjuster is interlocked instead to ensure that zero excitation is obtained after closing. In addition to this, the power supply of both tripping channels must be available and the key operated switch for blocking the excitation during commissioning and maintenance work (arranged in the voltage regulator cubicle) must be set to the position “Excitation not blocked”. With the field breaker being closed and the speed limits exceeded the pulse blocking signal to the gate control set disappears, the ramp function generator runs up thus building up the generator excitation provided that automatic voltage control (AUTO) has been selected. The run-up command is stored by a memory with remnant relay. When the speed drops below the limit values during shutdown, this initiates together with the status “Generator not loaded”.

KORBA SIMULATOR

380

• field breaker OFF command

• pulse blocking signal to the gate control set

• run-down of ramp function generator. Run-down of the ramp function generator may also be initiated during rated speed by the OFF state of the field breaker, when the breaker is tripped from the generator protective system. The speed criteria are monitored with respect to their importance. Presence of the criterion n< or absence of the criterion n > while the generator is loaded will be alarmed. Under excitation current control (MANUAL), no automatic excitation build-up is effected during start-up. When the field breaker is closed, the excitation current is at its lowest possible value = zero value approx. The desired excitation can be set on the set-point adjuster (lower/Raise pushbutton in UCB). During shut down of generator the field current set point adjuster receives a continuous LOWER command on tripping of the field breaker so that the set-point adjuster is set to the lower limit position. The tripping circuits for the de-excitation are provided twice for redundancy reasons. This should be complemented by corresponding safety in the power supply for the trip circuits; A de-excitation command from the generator protection system or a “Field breaker OFF” command from the control room energises relays K12 (system 1)/K22 (system 2) which seal in and start the time relays K13/K23, set to 0.5 s. Via relays K12/K22 the Thyristor set operating is driven to inverter operation thereby reversing the main exciter field winding voltages and thus reducing the Thyristor set output current to zero in less than half a second. The field discharge contractors K14/K24, energised by time relays K13 or K23 respectively, switch a field discharge resistor in parallel to the field winding of the main exciter and trip field breaker Q1 via its tripping coil. The field discharge resistor ensures that proper de-excitation is achieved even in the event of failure of the electronic de-excitation circuit. The field breaker is automatically tripped during generator shutdown by speed criteria as described above if not tripped earlier by the reverse power protection system. In emergencies, the field breaker can also be tripped manually via the generator protection system by actuating the emergency pushbutton on the control desk. In this case, also a turbine trip command is transmitted to the turbine control equipment. The OFF pushbutton for the field breaker is normally only connected for reset of the Iscamatic control module AS11 in the above cases. Should the OFF pushbutton be required to really trip the field breaker, interlocks must be provided with the generator

KORBA SIMULATOR

381

breaker and possibly with the station service supply breaker(s) to prevent de-excitation of loaded generator. Local non-electrical (mechanical) tripping of the field breaker is not permissible as the other essential de-excitation functions (field discharge by resistors, field suppression by overeater operation) are not tripped in this case. For emergency de-excitation a push-button or switch is locally provided (in the cubicle). Emergency de-excitation is possible also by tripping the MCB’s for the pulse power supply of the Thyristor sets. Mechanically closing of the field breaker is to be avoided also, as the sealed in relays in the tripping circuits would not drop out in this case. During short-circuit operation of the generator for setting of the generator protective equipment, the degree of excitation is adjusted by means of the excitation current regulator (MANUAL). During this mode of operation, a “MANUAL faulted” criterion available in the alarm system of the regulator can provisionally be used for tripping the field breaker. CHANGEOVER AUTO-MANUAL The THYRISIEM 04-2 voltage regulator includes a control system for automatic voltage control (AUTO) and an excitation current control system (MANUAL). The excitation current control system is provided for taking generator characteristics during commissioning and maintenance, for short-circuit operation of the generator during commissioning and for adjusting the excitation current in case of the automatic voltage regulator being faulty. This means that changeover from AUTO to MANUAL and vice versa is only required in exceptional cases. The manual changeover command is normally issued from the control room. Pushbuttons are also provided in the voltage regulator cubicle for commissioning and maintenance purposes. Push buttons AUTO, MATCH and MANUAL are provided for manual changeover. The MATCH pushbutton must be actuated prior to manual changeover. Following this, the RAISE, LOWER pushbuttons must be actuated for matching the output value of the set-point adjuster for MANUAL (on transition to MANUAL) or of the set-point adjuster for AUTO (on transition to AUTO) to the actual excitation state or to the generator voltage actual value. When the matched state is reached, the matching instrument in the control room indicates zero. Since different controlled variables are associated to the MANUAL to AUTO modes of operation, matching must not be effected by balancing the set-point adjuster position which are also indicated in the control room. Changeover to MANUAL or AUTO is only possible after the MATCH condition has been selected (interlocking circuit). Changeover to MANUAL or AUTO is also blocked from the regulator alarm system when the excitation current regulator or the automatic voltage regulator is faulty.

KORBA SIMULATOR

382

Automatic interrogation and evaluation of the matching stage and associated interlocking circuits have purposely not been provided since, in the event of faults changeover required for maintaining generator operation might be restricted inadequately. When all conditions for changeover are fulfilled, changeover is initiated by actuating pushbutton MANUAL or AUTO. The stored commands MATCH and MANUAL or AUTO are cancelled by the checkback signal “Gate control set MANUAL ON” or “Gate control set AUTO ON”. Changeover from AUTO to MANUAL and vice versa is initiated by a remanent relay module in the gate control set. Certain fault conditions in the automatic voltage control system initiate automatic changeover to MANUAL; This requires continuous automatic follow-up control of the excitation current set-point adjuster, i.e. on manual transition to MANUAL, a more or less balanced state already exists when MATCH is selected. Manual fine matching may still be carried out if necessary since automatic matching is blocked when MATCH is selected. FAULT INDICATIONS The following alarms are issued from the voltage regulator to the control room AVR fault; AVR, automatic changeover to MANUAL, AVR, loss of alarm voltage. The group alarm “AVR fault” collects the individual alarms; The initiating individual signal (s) is are stored and can be identified locally (in the voltage regulator cubicle) by means of LED’s. The indications are :- Power supply Auto; Generator voltage actual value; Thyristor set AUTO; Faulty over excitation. Cause automatic changeover to MANUAL unless changeover is not blocked due to a fault in the excitation current control (MANUAL). Changeover can also be checked if components of MANUAL or AUTO are not ready for operation. This blocking takes effect both for automatic and manual commands. Fault conditions initiating automatic changeover also cause the alarm “AVR’ automatic changeover to MANUAL” to be given. The Power Supply AUTO” alarm initiating automatic changeover occurs in the event of undervoltage in the stabilised 15 V power supply for the automatic voltage control and the associated gate control set. The alarm “Generator voltage actual value” is initiated either by tripping of the voltage transformer MCB for the generator voltage actual value or by the generator voltage actual value monitor.

KORBA SIMULATOR

383

Response of the current flow monitoring system initiates the “Thyristor set AUTO” alarm. AUTOMATIC VOLTAGE REGULATION [AUTO] The potentiometer R1 fed from the + 10 V stabilised voltage presets the base reference value, the standard corresponding 85 % rated voltage. Added to this is the variable value from the set-point adjuster as standard 0- 20 %, corresponding to a total setting range of 85 to 105 %. The output voltage of the subsequently arranged amplifier N2 amounts to 8.0 V at rated generator voltage. The comparison of the set-point with the actual value takes place at the input of the proportional amplifier N4. Furthermore the output of the amplifier N3 which sums up the influencing factors of the compensation (reactive current effect) and of the under and over excitation limiters, is also switched to this junction point. The result of these influences is that of an additional set-point. The inputs for the limiters as well as further free inputs of the amplifier N3 can be individually switched to this point (the switches are not shown). The compassion can be set to between 0 and approx. 10 % by a potentiometer. A cascade of a proportional - integral (PI) voltage regulator (amplifiers N4, N7) and a following excitation current regulator (amplifiers N14,N15) serves for dynamic is determined and amplified by amplifier N4; the gain is set at potentiometer R4. The integral function is provided by amplifier N7 and adjusted at potentiometer R7. The feedback resistor R8 determines the static gain. The amplifiers N9, N10 with high proportional gain (about 100) limit the positive and negative output voltage of the amplifier N8 and thus the input signal to the excitation current regulator, depending on the setting of the potentiometers R9, R10. Negative output voltage of the P1 voltage regulator (output of N8) results in a positive set-point value to the input of the excitation current proportional (P-) regulator N 15. This P-regulator compares the positive set-point value against a negative actual value signal from amplifier by potentiometer R15. Under steady-state conditions, the set-point and actual value signals have approx. The same amount, the difference setting to a value which multiplied by the gain of N15 result in the required signal going to the gate control set (output of N15). Negative voltage to the gate control set generates firing angles < 90 deg. Thus supplying power to the field winding of the main exciter (controlled rectifier operation) Positive voltage generates firing > 90 deg. Thus reversing the Thyristor set output voltage and drawing power from the field winding of the exciter, resulting in the current falling towards zero (inverter operation).

KORBA SIMULATOR

384

KORBA SIMULATOR

385

The relay K15 switches a positive voltage to a limiting input of amplifier N15 during de-excitation resulting in an equal positive constant input voltage to the gate control set. The Thyristor set output current then drops in less than half a second to zero, the generator voltage - or generator current in the case of a short-circuit - follows with a delay corresponding to the relevant generator time constant. As soon as the current reaches zero the Thyristor are blocked and the reversed voltage on the field winding disappears. This de-excitation process by inverter operation is shown below. The excitation current actual value = output current of the Thyristor sets is sensed by two transducers connected to separate shunt resistors, i.e. one combination each of shunt resistor plus transducer is provided for the automatic voltage regulator (AUTO) and for the MANUAL control system. The matching amplifier following the transducer for automatic voltage control transmits the current actual value to the input of amplifier N14. FIELD FORCING With the automatic voltage regulation calling for maximum excitation of the generator, the Thyristor set initially supplies a higher voltage to the field winding of the main exciter than that actually required under steady-state conditions, until the actual current in the main exciter field winding has reached the excitation current set-point coming from the PI-voltage regulator (output of N8). Overdriving the voltage to the field winding of the main exciter reduces the time required for building up the corresponding current in same field winding and thus improves the exciter response, for a large control action. The overdriving function - also refereed to as field forcing - also becomes effective in the case of smaller control operations. This effect is achieved by the proportional excitation current regulator N15 together with proper setting of the control limits.

KORBA SIMULATOR

386

The maximum possible output voltage of the Thyristor set UA MAX is determined by a limit set as required in the gate control unit; i.e. the minimum delay angle for rectifier operation. Because of the high gain of the current regulator N15, a small increase of the set point is sufficient to reach UA MAX. The maximum output current IA MAX is determined by the maximum possible excitation current set-point from the PI-voltage regulator; this set-point is limited by amplifier N10 with respect to a reference voltage coming from ramp function generator N11, the function of which is explained below. This reference voltage is corresponding with the setting on potentiometer R11. Overdriving also becomes effective in case of downward control actions. The maximum possible reversed output voltage again is determined by a limit set (to a standard value) in the gate control unit, i.e. the maximum delay angle for inverter operation,. The positive signal from the PI-voltage regulator to the excitation current regulator is limited by amplifier N9 according to the setting of potentiometer R9 to a small value; this (standard) value is selected to make possible maximum reversed Thyristor output voltage during downward regulation through the whole current range, i.e. also at small actual current values. At the beginning of a generator start-up cycle, the output voltage of the ramp function generator N11 is zero so that the Thyristor set output current is limited to zero. When a speed value just short of synchronisation is reached, the ramp function generator gets its input voltage to run up to its maximum value within approx. 20. Thus due to the gradual enabling of the current, the generator receives its voltage within a few seconds without overshooting. On shutdown of the machine the ramp function generator runs back to output zero after removing the run-up command. Failure of diodes in the rotating rectifier between the main exciter and the generator field winding reduces its load capacity. The control panel in the regulator cubicle includes a switch by means of which the normal field - forcing value IA MAX of 1.5 times rated excitation can be limited to 1.1 times rated output by means of which the field - forcing limitation is reduced. The limitation to 1.1 times rated excited also becomes effective when the generator is not loaded; Field -forcing limitation, i.e. limitation of the Thyristor set output current to IA MAX ‘ is monitored by a limit monitor which senses the output current of the Thyristor set and its pick-up value being adjusted to 1.1 x IA MAX. Response of this limit monitor initiates automatic changeover to the MANUAL control system.

KORBA SIMULATOR

387

MANUAL CONTROL SYSTEM The MANUAL control system incorporates a proportional action amplifier for closed-loop control of the Thyristor set output current = excitation current of the main exciter. This control system is provided for commissioning and maintenance work (particularly for operating the generator short-circulated) and maintaining operation when the automatic voltage regulator is disturbed. The current actual value for the control system is transmitted via a shunt from transducer U1 and matching amplifier N6. The set-point value is supplied by a motor-operated potentiometer with a setting range which normally goes from the remanence value to 1.1 times rated excitation. The range is adjusted by a potentiometer R6 of matching amplifier N6. The set-point and actual values are compared at the input of proportional amplifier N1. The output of amplifier N1 supplied the input voltage for the gate control set via amplifier N2 (gain = +1). Amplifier N3, N4 with high proportional gain (approx. 100) limit the positive and negative output voltage of amplifier N2 and thus the input signal of the gate control set in accordance with the setting of potentiometers R2,R3. Potentiometer R3 is set in accordance with the Thyristor set output voltage actually required (approx. 1.1. times rated excitation).Potentiometer R2 for the maximum inverter voltage is set to a standard value. For de-excitation, the positive output voltage is changed over from potentiometer R2 to amplifier N4 by means of relay K1 thus ensuring an equal positive constant input voltage of the gate control set. On tripping of the generator from the grid, the current regulator would maintain the excitation current on a level corresponding to the preceding load operation which would result in generator over voltage. This is prevented by two measures. Amplifier N5 limits the generator overvoltage to a response value pre-set by potentiometer R4 this response value must be slightly higher than the maximum generator voltage to be expected during operation (e.g. 106 %). In addition to this, value which corresponds to the station supply load. CONTROL OF SET-POINT ADJUSTERS Each of the two control systems AUTO and MANUAL has its own 24 V d.c. motor-operated set-point adjuster; Two separate output drivers are provided for supplying the motor in both directions of rotation. When one of the end positions is reached, the output voltage of the set-point potentiometer blocks the relevant driver by a limit value monitor. In addition, the drive is protected by a slip-clutch. The set-point

KORBA SIMULATOR

388

potentiometer can also be adjusted manually without electric control by means of a knob provided on the module front panel. The LOWER or RAISE signals transmitted from the control room act on the OR function in the driver input. The driver are mutually interlocked via blocking inputs. Furthermore , response of the under excitation limiter blocks the RAISE driver of the AUTO set point adjuster. The inputs of the drivers for the MANUAL set point adjuster are also influenced by the follow -up control system (refer to chapter 11). Furthermore, U5 when the generator is disconnected from the grid and during de-excitation. Limit monitor U5 reduces the set point value to a value which approximately corresponds to the station-service power requirements of the unit. The activity of limit monitor U5 during shutdown of the generator was provided to assist the operator who must carry out a fine adjustment in accordance with the generator voltage. During de-excitation, the set-point adjuster is moved to the lower end position. Together with the LOWER / RAISE commands from the control room, the corresponding commands from the automatic synchroniser and the local control panel in the regulator cubicle are applied through or gates. The LOWER/RAISE commands are transmitted to both set-point adjusters to support follow-up control. During matching, only the commands to the set-point adjuster to be matched are enabled. MATCHING FOLLOW UP CONTROL OF THE EXCITATION CURRENT SET-POINT ADJUSTER The excitation current regulator (MANUAL) is mainly used during commissioning and maintenance work and in exceptional cases when faults occur in the automatic voltage control system. For this purpose, the setting of the excitation current set-point adjuster must be matched to the actual excitation state before changeover to MANUAL. In some special fault cases, changeover to manual is initiated automatically. Therefore matching is to be carried out continuously by means of an automatic follow-up control system. It must be taken into consideration that the excitation current may be subject to considerable transient variations during faults. For this reason, a clear design of the matching and follow-up control circuits is more essential than high accuracy. Under steadystate conditions, the matching and follow circuits ensure, that the sustained and transient change of the excitation caused by changeover practically are to be neglected. Changeover from control of the generator voltage (AUTO) to the excitation current regulator (MANUAL requires the excitation current set-point adjuster to be set to a position in which a set point corresponding to the actual excitation current is supplied.

KORBA SIMULATOR

389

The output values of the excitation current regulators for AUTO and MANUAL are compared for that purpose by amplifier N1. The difference signal acts on the matching instrument in the control room and on the input of the three term controller U1 for follow-up control of the excitation current set-point adjuster. The set-point value of the excitation current regulator is suitably adjusted for changeover when the difference signal at the outputs of amplifiers N1 and N2 is zero. The difference signal at the output of amplifier N2 is connected to a proportional amplifier U1 N1 with adjustable gain and substantial smoothing in the three term controller. The input of the limit monitors U1U1/U1U2 for Lower/Raise outputs with fixed response thresholds are connected to the output of amplifier U1N1. The difference value which just initiates response of the three-term controller is determined by the proportional gain of U1N1. In the case of larger differences, the response time of the limit monitors is shortened as a function of the increasing difference; refer to the AVR diagrams “response mode” and “response time”. The values of the gain and smoothing time constant of amplifier U1N1 are selected so that on the one hand sufficient accuracy and fast correction of the deviation is ensured, and on the other hand the switching frequency of the set-point potentiometer is sufficiently limited. The circuits of the three-term controller are shown simplified without the components which ensure a minimum pulse time and stabilisation of the follow-up control (feedback). When the generator is connected in parallel with grid, the LOWER /RAISE commands from the control room are simultaneously transmitted to the AUTO and MANUAL set point adjusters. This reduces the setting time of the follow-up control system. For monitoring of the follow-up control system, the difference signal behind amplifier N2 is monitored by limit monitors. Response of a monitor initiates delayed signalling. To prevent disturbing changes in excitation during changeover from MANUAL to AUTO, the output voltage of the generator voltage regulator must be adjusted according to the actual excitation level i.e. the excitation current set-point delivered by the generator voltage regulator must be adapted to the excitation current actual value. The excitation current set-point value is made up by two components:

• The voltage of a proportional amplifier N4* and

• The voltage of an integrator N7* During steady-state conditions, the output voltage of amplifier N4* is zero and the output voltage of integrator N7* delivering the set-point to the excitation current regulator of the AUTO system. This means that the AUTO set-point adjuster must be so adjusted that the output voltage of amplifier N4* becomes zero, whereas the output voltage of integrator N7* is essentially to be adjusted to the excitation current actual value. The first task is solved by applying the output voltage of amplifier N4* to the input of amplifier N2 feeding the matching instrument. The instrument reading is to be adjusted to zero by LOWER/RAISE commands from the control room.

KORBA SIMULATOR

390

KORBA SIMULATOR

391

UNDEREXCITATION LIMITER The underxcitation limiter automatically prevents too low excitation of the generator. A reduction of the excitation may, for instance, occur under influence of the automatic voltage regulator when the system voltage rises during low-load operation. A reduction of the excitation may also result from a faulty operation, e.g. the tapping switch of the main transformer. Fig. 1 illustrates the response value for a generator with an assumed maximum rotor

displacement angle of 75o. This response value includes an adequate safety margin above the stability limit. The safety margin allows for transient phenomena due to major system disturbances and switching operations. This safety margin is of particular importance in the event of short-circuits occurring close to the generator terminals, since the rotor displacement angle increases rapidly in this case. The final value of the rotor displacement angle on clearance of the fault is the decisive factor for maintaining stability. This angle will increase with increases of the steady-state initial value of the rotor displacement angle or decreases of the excitation.

OVEREXCITATION LIMITER Reduction of the system voltage due to increased reactive power requirements, switching operation or disturbances cause the voltage regulator to increase the generator excitation in order to maintain a constant generator voltage. Major system voltage reductions may result in a thermal overloading of the exciter and generator

KORBA SIMULATOR

392

rotor unless the operator presets a lower set-point for the generator voltage or changes the ratio of the unit transformer. In such a case the overexcitation limiter limits the generator excitation by automatically reducing the generator voltage. The excitation current if is measured through a current transformer with shunt and compared with a set-point. When the excitation current exceeds the set-point value, a signal appears at the output of the overexcitation limiter. The resulting signal in the input amplifier of the voltage regulator causes the excitation to decrease accordingly. The overexcitation limiter has a response time inversely proportional to the difference between the actual value and the response value. The shortest response time should be coordinated with the time setting of the backup protection of the generator (Fig.2). The voltage regulator keeps the generator voltage constant independent of the generator frequency. Excitation of the generator with excessive under frequency values is prevented by speed-dependent enabling of the excitation at a speed value of 0.95 p.u. approx. or by blocking at a speed value of less than 0.90 p.u. approx. This means that the excitation equipment permits excited operation of the generator with frequency deviations up to 0.1 p.u. below nominal frequency. The magnetic flux of the unit transformer is directly proportional to the terminal voltage and inversely proportional to the frequency, i.e. proportional to the ratio terminal voltage/frequency. The maximum under frequency value mentioned above may cause a rise of the unit transformer flux. by 0.1 p.u. as against the nominal value. A further increase is possible depending on the permissible setting range of the generator voltage and even higher magnetic flux values are obtained when fault conditions of the voltage regulator are taken into consideration. The conditions described above basically also apply to the magnetic flux of the generator. Excessive magnetic flux increases thermal stressing of the unit transformer and of the generator. The function of the V/Hz limiter is to issue a signal to the voltage regulation loop when a present V/Hz limit value is exceeded and to reduce this value to the permissible limit. For this purpose the V/Hz limiter includes an element for measuring the frequency, comparing the frequency value against the generator voltage value and evaluating a correction signal. The action of the V/Hz limiter is frequently restricted to operation with the generator being disconnected from the grid. POWER SYSTEM STABLISER GENERAL The power system stabilizer is an integral part of the voltage regulator. The stabilizer has the function of damping any turboset power fluctuations following power supply failures or in service switching operations in order to increase operational reliability

KORBA SIMULATOR

393

and security of supply. Simultaneously the stabilizer generally attains smoother turboset behavior in normal power supply operation. Mode of Operation Given power system structures in combination with certain generator output levels, and also disturbances in the power supply, can frequently produce small power system short circuit currents. In addition, the higher utilization factor (increased output) of modern generators with correspondingly high excitation responses reduces the inherent damping of generators on line. This is understandable in view of the fact that the damping torques are largely dependent on the geometry of the rotor, whereas the increased output of the generators was primarily achieved by more intensive cooling of the rotor; consequently the rotor dimensions did not increase in proportion to output. These two trends, reduction of the ratio of power system short circuit current to generator output and less effective damping of the large generators, lead as a whole to a greater tendency toward fluctuation of the overall generator power supply system. A system stabilization of turbo sets can as a rule be achieved through reduction of the proportional gain of the turbine governing system. Control action influencing the generator excitation system is however a significantly better means of increasing damping. For reasons of technology, the inherent damping in the damping circuits of the generator, which is proportional to slip, cannot directly be increased by inputting additional damping signals. Such signals to improve the stability of the turboset on line must pass through various existing control elements and thus be exactly matched in their time response. Additional damping through feed forward to the signal comparison circuit of the generator voltage regulator with its integral power system stabilizer can be made large enough to override disturbances in other control circuits. The stabilizer for power systems of differing structures operates on signals derived from active power changes (actual power values) and acts on the generator excitation, system by inputting an additional damping signal in the proper phase relation. The power system stabilizer increases oscillation damping in a frequency range from 0.05 Hz to 2.5 Hz. For this purpose, the monitored output power is fed to the inputs of two 2-point vector identifiers, each having one in phase and one phase-shifted output. The output signals in cross-connected opposed-phase pairs are fed via matching elemets to one summing

KORBA SIMULATOR

394

element per pair; the output signals from the two summing elements are fed through one matching element per signal to an additional summing element. The resultant output signal is transmitted via a noise suppression element and a limiter as an additional input to the signal comparison circuit of the voltage regulator.

Filename: Volume1 Directory: D:\Backup-CD\Manuals\500MW Template: C:\Documents and Settings\SIMULATOR\Application

Data\Microsoft\Templates\Normal.dot Title: Subject: Author: Simulator Keywords: Comments: Creation Date: 12/8/2005 3:06 PM Change Number: 93 Last Saved On: 4/28/2006 12:05 AM Last Saved By: user Total Editing Time: 1,579 Minutes Last Printed On: 7/14/2006 11:42 AM As of Last Complete Printing Number of Pages: 394 Number of Words: 95,502 (approx.) Number of Characters: 477,512 (approx.)