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.
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
PLANT SIMULATION
AND
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)
• 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
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.
• 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
• 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
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
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
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),
• 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.
BOILER
AND
AUXILIARIES
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.
• 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.
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.
TECHNICAL SPECIFICATION OF 500 MW BOILER
MAIN BOILERGENERAL 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 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
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
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
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
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.
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.
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.
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)
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)
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
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.
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 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.
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
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 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
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.
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
INTRODUCTIONThe 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.
TILTING TANGENTIAL FIRING SYSTEM
GENERALIn 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.
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
