REPORT ON VOCATIONAL TRAINING AT MEJIA THERMAL POWER STATION BASUDEB SHIT JADAVPUR UNIVERSITY (MECH)
Dt 14/05/14 – 03/06/14
This project report has been prepared in fulfillment if industrial training to be carried out in third year of our four year B.E course. For preparing the project p roject report, we visited MEJIA THERMAL POWER STATION under DAMADAR VALLEY CORPORATION during the suggested su ggested duration for the period of 21 days, to avail the necessary information. The blend of learning le arning and knowledge acquired during our practical studies at the company is presented prese nted in this project report. The rationale behind visiting the power plant and preparing the project is to study the mechanical operation in details, electrical overview, various cycles and processes of power generation and details of control and instrumentation required in thermal power plant.
I take this opportunity to express my profound gratitude and deep regards to Mr. P.K. Dubey for his exemplary guidance, monitoring and constant encouragement throughout the course of this thesis. The blessings ,help and guidance given by him time to time shall carry me a long way in the journey of life on which I am about to embark. I also take this opportunity to express a deep gratitude to MEJIA THERMAL POWER STATION, DVC, for their cordial support, valuable information and guidance, which helped me in completing this task in various stages. I am also thankful to Mr. N. C. Pal, the Dy. Chief engineer(mech), MTPS for his guidance and constant support throughout the training period. I am also thankful to the chief engineer and project head, MTPS and assistant director, HR for providing me opportunity to carry out my vocational training in MTPS. I am obliged to staff members of MTPS, DVC for the valuable information and time provided by them in their respective fields. I am grateful for their cooperation during the period of my assignment.
Damodar Valley Corporation was established on 7th July 1948.It is the most reputed company in the eastern zone of India. DVC in established on the Damodar River. It also consists of the Durgapur Thermal Power Plant in Durgapur. The MTPS under the DVC is the second largest thermal plant in West Bengal. It has the capacity of 2340MW with 4 units of 210MW each, 2 units of 250MW each & 2 units of 500 MW each. With the introduction of another two units of 500MW that is in construction it will be the largest in West Bengal. Mejia Thermal Power Station also known as MTPS is located in the outskirts of Raniganj in Bankura District. It is one of the 5 Thermal Power Stations of Damodar Valley Corporation in the state of West Bengal. The total power plant campus area is surrounded by boundary walls and is basically divided into two major parts, first the Power Plant area itself and the second is the Colony area for the residence and other facilities for MTPSs ͛ employees.
TECHNICAL SPECIFICATION OF MTPS INSTALLED CAPACITY: 1. Total number of Units : - 4 X 210 MW(unit 1 to 4) with Brush Type Generators, 2 X 250 MW(unit 5 and 6) with Brush less Type Generators, 2*500 MW(unit 7 and 8) Generators. 2. Total Energy Generation: - 2340 MW 3. Source of Water: - Damodar River 4. Sources of Coal: - B.C.C.L and E.C.L, also imported from Indonesia In a Thermal Power generating unit, combustion of fossil fuel (coal, oil or natural gas) in Boiler or fissile element (uranium, plutonium) in Nuclear Reactor generates heat energy. This heat energy transforms water into steam at high pressure and temperature. This steam is utilized to generate mechanical energy in a Turbine. This mechanical energy, in turn is converted into electrical energy with the help of an Alternator coupled with the Turbine. The production of electric energy utilizing heat energy is known as thermal power generation. The heat energy changes into mechanical energy following the principle of Rankin reheatregenerative cycle and this mechanical energy transforms into electrical energy based on Faraday’s laws of electromagnetic induction. The generated output of Alternator is electrical power of three-phase alternating current (A.C.). A.C. supply has several advantages over direct current (D.C.) system and hence, it is preferred in modern days. The voltage generated is of low magnitude (14 to 21 KV for different generator rating) and is stepped up suitably with the help of transformer for efficient and economical transmission of electric power from generating stations to different load centers at distant locations.
OVERVIEW OF THERMAL POWER PLANT A thermal power plant continuously converts the energy stored in the fossil fuels(coal, oil, natural gas) into shaft work and ultimately into electricity. The working fluid is water which is sometimes in liquid phase and sometimes in vapor phase during its cycle of operation. Energy released by the burning of fuel is transferred to water in the boiler to generate steam at high pressure and temperature, which then expands in the turbine to a low pressure to produce shaft work. The steam leaving the turbine is condensed into water in the condenser where cooling water from a river or sea circulates carrying away the heat
released during condensation. The water is then fed back to the boiler by the pump and the cycle continues.
MECHANICAL OPERATION COAL HANDLING PLANT Generally most of the thermal power plants uses low grades bituminous coal. The conveyer belt system transports the coal from the coal storage area to the coal mill. Now the FHP (Fuel Handling Plant) department is responsible for converting the coal converting it into fine granular dust by grinding process. The coal from the coal bunkers. Coal is the principal energy source because of its large deposits and availability. Coal can be recovered from different mining techniques like shallow seams by removing the over burnt expose the coal seam underground mining The coal handling plant is used to store, transport and distribute coal which comes from the mine. The coal is delivered either through a conveyor belt system or by rail or road transport. The bulk storage of coal at the power station is important for the continues supply of fuel. Usually the stockpiles are divided into three main categories. live storage emergency storage long term compacted stockpile. The figure below shows the schematic representation of the coal handling plant. Firstly the coal gets deposited into the track hopper from the wagon and then via the paddle feeder it goes to the conveyer belt#1A. Secondly via the transfer port the coal goes to another conveyer belt#2B and then to the crusher house. The coal after being crushed goes to the stacker via the conveyer belt#3 for being stacked or reclaimed and finally to the desired unit. ILMS is the inline magnetic separator where all the magnetic particles associated with coal get separated.
COAL HANDLING PLANT
COAL MILL OVERVIEW
COAL MILL- AB
WATER TREATMENT PLANT Raw water supply Raw water received at the thermal power plant is passed through Water -
Treatment Plant to separate suspended impurities and dissolved gases including organic substance and then through De-mineralized Plant to separate soluble impurities. De-aeration In this process, the raw water is sprayed over cascade aerator in which water flows downwards over many steps in the form of thin waterfalls. Cascading increases surface area of water to facilitate easy separation of dissolved undesirable gases (lik e hydrogen sulphide, ammonia, volatile organic compound etc.) or to help in oxygenation of mainly ferrous ions in presence of atmospheric oxygen to ferric ions. These ferric ions promote to some extent in coagulation process. -
Coagulation -
Coagulation takes place in clariflocculator. Coagulant destabilises suspended solids and agglomerates them into heavier floc, which is separated out through sedimentation. Prime chemicals used for coagulation are alum, poly-aluminium chloride (PAC). Filtration
Filters remove coarse suspended matter and remaining floc or sludge after coagulation and also reduce the chlorine demand of the water. Filter beds are developed by placing gravel or coarse
anthracite and sand in layers. These filter beds are regenerated by backwashing and air blowing through it. Chlorination:
Neutral organic matter is very heterogeneous i.e. it contains many classes of high molecular weight organic compounds. Humic substances constitute a major portion of the dissolved organic carbon from surface waters. They are complex mixtures of organic compounds with relatively unknown structures and chemical composition.
DM (De-mineralized Water) Plant In De-mineralized Plant, the filter water of Water Treatment Plant is passed through the pressure sand filter (PSF) to reduce turbidity and then through activated charcoal filter (ACF) to absorb the residual chlorine and iron in filter water.
BOILER AND AUXILLURIES BOILER
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Working principle of Boiler (Steam Generator): In Boiler, steam is generated from de-mineralized water by the addition of heat. The heat added has two parts: sensible heat and latent heat. The sensible h eat raises the temperature and pressure of water as well as steam. The latent heat converts water into steam (phase change). This conversion is also known as boiling of water, which is dependent on pressure and corresponding temperature. Thermodynamically, boiling is a process of heat addition to water at constant pressure & temperature. The quantity of latent heat decreases with increase in pressure of water and it becomes zero at 221.06 bars. This pressure is termed as critical pressure. The steam generators are designated as sub-critical or super critical based on i ts working pressure as below critical or above critical pressure. T he steam, thus formed is dry & saturated. Further, addition of heat raises the temperature and pressure of steam, which is known as superheated steam. The differential specific weight between steam and water provides the driving force for natural circulation during the steam generation process. This driving force considerably reduces at pressure around 175 Kg/cm2 and is not able to overcome the frictional resistance of its flow path. For this, forced or assisted circulation is employed at higher sub-critical pressure range due to the reason of economy. But, at supercritical pressures and above, circulation is forced one (such boiler is called once through boiler). Important parts of Boiler & their functions: Economizer:
Feed water enters into the boiler through economizer. Its function is to recover residual heat of flue gas before leaving boiler to preheat feed water prior to its entry into boiler drum. The drum water is passed through down-comers for circulation through the water wall for absorbing heat from furnace. The economizer recirculation line connects down-comer with the economizer inlet header through an isolating valve and a non-return valve to protect economizer tubes from overheating caused by steam entrapment and starvation. This is done to ensure circulation of water through the tubes during initial lighting up of boiler, when there is no feed water flow through economizer. Drum:
Boiler drum is located outside the furnace region or flue gas path. This stores certain amount of water and separates steam from steam-water mixture. The minimum drum water level is always maintained so as to prevent formation of vortex and to protect water wall tubes (especially its corner tubes) from steam entrapment / starvation due to higher circulation ratio of boiler. The secondary stage consists of two opposite bank of closely spaced thin corrugated sheets which direct the steam through a tortuous path and force the remaining entrained water against the corrugated plates. Since, the velocity is relatively low, this water does not get picked up again but runs down the plates and off the second stage lips at the two steam outlets. From the secondary separators, steam flows uniformly and with relatively low velocity upward to the series of screen dryers (scrubbers), extending in layers across the length of the drum. These screens perform the final stage of separation. Super-heater: Super-heaters (SH) are meant for elevating the steam temperature above the saturation temperature in phases; so that maximum work can be extracted from high energy (enthalpy) steam and after expansion in Turbine, the dryness fraction does not reach below 80%, for avoiding Turbine blade erosion/damage and attaining maximum Turbine internal efficiency. Steam from Boiler Drum passes through primary super-heater placed in the convective zone of the furnace, then through platen super-heater placed in the radiant zone of furnace and thereafter, through final super-heater placed in the convective zone. The superheated steam at requisite pressure and temperature is taken out of boiler to rotate turbo-generator. Re-heater: In order to improve the cycle efficiency, HP turbine exhaust steam is taken back to boiler to increase temperature by reheating process. The steam is passed through Re-heater, placed in between final super-heater bank of tubes & platen SH and finally taken out of boiler to extract work out of it in the IP and LP turbine. De-super-heater (Attemperator) : Though super-heaters are designed to maintain requisite steam temperature, it is necessary to use de-super-heater to control steam temperature. Feed water, generally taken before feed water control station, is used for de-superheating steam to control its temperature at desired level. Drain & Vent:
Major drains and vents of boiler are (i) Boiler bottom ring header drains, (ii) Boiler drum drains & vents, (iii) Super-heater & Re-heater headers drains & vents, (iv) De-super-heater header drains & vents etc. Drains facilitate draining or hot blow down of boiler, as and when required; while vents ensure blowing out of air from boiler during initial lighting up as well as facilitate depressurizing of boiler. The continuous blow down (CBD) valve facilitates reduction in contaminant concentration in drum water and also complete draining of drum water. The intermittent blow down (IBD) emergency blow down (EBD) valve helps to normalize the excess drum water level during emergency situation.
Soot Blower :
It is an on load arrangement of cleaning the accumulation of deposits on boiler heating surfaces resulting from combustion of coal and to a relatively smaller extent from oil. Superheated steam, taken out from main steam line and reduced to pressure around 30 kg/cm2, is passed through different types of blowers placed at different location of boiler to clean the deposits for effective heat transfer through the metal surface. The different types of blowers are (a) Long retractable soot blower for cleaning the superheater, reheater zones, (b) Wall blower for cleaning the furnace wall and (c) Air pre-heater soot blower for cleaning air preheater baskets especially during oil firing to prevent damage of the basket materials from oil firing. Air pre-heater :
The air pre-heater absorbs waste heat from flue gas and then transfers this heat to incoming cold air by means of continuously rotating heat transfer elements of specially formed metal plates known as Basket (rotary re-generative air pre-heater) or through stationery tubes (recuperative or tubular air pre-heater). In re-generative type, thousands of high efficiency elements are spaced and compactly arranged with sectors shaped compartments of a radially divided cylindrical shell called the rotor. The housing surrounding the rotor is provided with duct connections at both ends and is adequately sealed by radial and circumferential sealing members forming an air passage through one or two sectors of the pre-heater and a gas passage through the other sector. As the rotor slowly revolves the mass of elements through the gas and air passages, heat is absorbed by the element surfaces passing through the hot gas stream; then as these same surfaces are carried through the air stream they release the stored up heat – thus greatly increasing the temperature of the incoming air. In the recuperative type, the flue gas is passed through the tubes and the air is passed over outer side of the tubes. TECHNICAL DATA OF THE BOILER Radiant, Reheat, Natural circulation, Single Type Drum, Balanced drift, Dry bottom, Tilting tangential, Coal and oil fired with DIPC (Direct Ignition of Pulverized Coal) system.
Width Depth Volume Fuel heat input per hour Designed pressure Super-heater outlet pressure Low temp SH (horizontally spaced) Platen SH(Pendant Platen) Final SH(Vertically spaced)
FURNACE 13868 mm. 10592 mm. 5240 mm. 106 kcal 175.8 kg per sq. cm 155 kg per sq. cm 2849 sq. m –total heating surface area 1097 sq. m –total heating surface area 1543 sq. m –total heating surface area ATTEMPERATOR
Type no. of stages Spray medium
Spray One feed water from boiler feed pump
RE-HEATER Vertical spaced 2819 sq. m. Burner tilt and excess air
Type Total H.S. area Control
ECONOMISER Type Total H.S. area
Plain tube 6152 sq.m.
ASH HANDLING PLANT A large quantity of ash is, produced in steam power plants using coal. Ash produced in about 10 to 20% of the total coal burnt in the furnace. Handling of ash is a problem because ash coming out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by some poisonous gases. It is desirable to quench the ash before handling due to following reasons: 1. Quenching reduces the temperature of ash. 2. It reduces the corrosive action of ash. 3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will disintegrate. 4. Quenching reduces the dust accompanying the ash. Flyash is collected with an electrostatic precipitator (ESP). Bottom ash hopper (BAH)
It is a water filled hopper placed below the furnace water wall ring headers to receive bottom ash generated out of combustion of coal. BAH is generally of two types – Dry Bottom and Wet Bottom. In Dry Bottom type,the BAH is kept empty (i.e. not water filled) prior to receiving Bottom Ash and quenched by separate spray arrangement from inside. In this system continuous ash evacuation and disposal take place through ash handling devices like Scrapper Conveyor (Ex. DTPS unit # 4) or Pneumatic Dry Ash conveying system. In the Wet Bottom type, the BAH is filled with water for quenching, disintegrating and collecting the Bottom Ash in water mixed condition. In this case ash evacuation & disposal is carried out periodically (ones in a shift of 8 hrs., Ex. CTPS units, DTPS U#3). There is a water seal, called Trough Seal placed in-between furnace bottom ring header and the BAH. The water seal allows the Boiler to expand downwards. A level of water is maintained in the Trough Seal for proper sealing of Boiler against air infiltration. The Bottom ash hopper is generally filled with water, which reduces the temperature of deposited hot ash and clinkers. Sudden quenching also helps in disintegration of clinkers. The deposited ash in the form of granules & clinkers is taken out periodically with the help of either a moving chain grate scrapper or by gravity to the clinker grinder chamber. The clinker grinder breaks huge lumps of ash / clinkers into small sizes which are easily carried out with the help of hydro-ejector to a
slurry sump. The hydro-ejector is a water jet pump designed to provide the power to transport slurry i.e. a mixture of small sized solid lump of ash / clinker and water to slurry sump through pipeline. The hydro-ejector consists of convergent primary & secondary nozzles. High pressure water is passed through the primary nozzle. The action of high pressure water creates high suction at venturi throat of the secondary nozzle, which draws accumulated slurry at the outlet chamber of the grinder. The total mixture then passes through the divergent part of the nozzle, which converts the energy of motion (velocity head) back i nto pressure energy to facilitate movement of the mixture to the slurry sump. ELECTROSTATIC PRECIPITATOR The principal components of an ESP are 2 sets of electrodes insulated from each other. First set of rows are electrically grounded vertical plates called collecting electrodes while the second set consists of wires called discharge electrodes.
The above figure shows the operation of an ESP. the negatively charged fly ash particles are driven towards the collecting plate and the positive ions travel to the negatively charged wire electrodes. Collected particulate matter is removed from the collecting plates by a mechanical hammer scrapping system. Induced draft fan (ID fan) –
Induced draft represents the system where air or products of combustion are driven out after combustion at boiler furnace by maintaining them at a progressively increasing sub atmospheric pressure. This is achieved with the help of induced draft fan and stack. Induced draft fan is forward curved centrifugal (radial) fan and sucks the fl y-ash laden gas of temperature around 125°C out of the furnace to throw it into stack (chimney). The fan is connected with driving motor through hydro-coupling or with variable frequency drive
(VFD) motor to keep desired fan speed.
No. of boiler Type Medium handled Location Orientation
TECHNICAL DATA OF I.D. FAN 3 Radial Flue gas Ground floor Suction—Vertical/45 degree to Horizontal Delivery—Bottom Horizontal.
Forced draft fan (FD fan) -
Forced draft represents flow of air or products of combustion at a pressure above atmosphere. The air for combustion is carried under forced draft conditions and the fan used for this purpose is called Forced Draft (FD) fan. It is axial type fan and is used to take air from atmosphere at ambient temperature to supply air for combustion, which takes entry to boiler through wind box. In all units except Durgapur TPS Unit #4, this fan also supplies hot /cold air to the coal mills. The output of fan is controlled by inlet vane / blade pitch control system.
No. of boiler Type Medium handled Location Orientation
3 Radial Clean air Ground floor 45 degree horizontal, delivery –bottom horizontal
Primary air fan (PA fan) or Exhauster fan:
The function of primary air is to transport pulverized coal from coal mill to the furnace, to dry coal in coal mill and also to attain requisite pulverized coal temperature for ready combustion at furnace. In some units like Chandrapura TPS unit 1, 2 & 3, the exhauster fan sucks pulverized coal and air mixture from coal mill and sends it to the furnace.
No. of boiler Type Medium handled Location Orientation
Technical data of the P.A. Fan 3 Radial Hot air Ground floor Suction—Vertical/45 degrees to Horizontal Delivery—Bottom Horizontal.
Coal mill or pulveriser:
Most efficient way of utilizing coal for steam generation is to burn it in pulverized form. The coal is pulverized in coal mill or pulverizer to fineness such that 70-80% passes through a 200 mesh sieve. The factors that affect the operation of the mill or reduce the mill output are: Grindability of coal: Harder coal (i.e. coal having lower hard-grove index (H.G.I.)) reduces mill output and vice versa. Moisture content of coal: More the moisture content in coal, lesser will be the mill output & vice versa. Fineness of output: Higher fineness of coal output reduces mill capacity. Size of coal input: Larger size of raw coal fed to the mill reduces mill output. Wear of grinding elements: More wear and tear of grinding elements reduces theoutput from mill.
Fuel oil system :
In a coal fired boiler, oil firing is adopted for the purpose of warming up of the boiler or assisting initial ignition of coal during introduction of coal mill or imparting stability to the coal flame during low boiler load condition. Efficient or complete combustion of the fuel oil is best achieved by atomizing oil by compressed air for light oi l (LDO) or by steam for heavy oil (HFO) in order to have proper turbulent mixing of oil with combustion air. Use of HFO is beneficial with respect to LDO in view of its lower cost and saving in foreign exchange. The oil burners and igniters are the basic elements of oil system. Oil is supplied by light oil pump or by heavy oil pump through oil heater. Steam heater reduces the viscosity of heavy oil and aids flow ability as well as better atomization. The oil burners are located in the compartmented corner of wind boxes, in the different elevation of auxiliary air compartments, sandwiched between the coal burner nozzles. Each oil burner is associated with an igniter, arranged at the side.
FUEL OIL SYSTEM
STEAM TURBINE A steam turbine is a prime mover which continuously converts the energy of high pressure, high temperature steam supplied by the boiler into shaft work with low pressure, low temperature steam exhausted to a condenser.
210 MW (KWU) steam turbine (Mejia TPS U # 1,2,3 & 4):
HP turbine inlet steam: 147 kg/cm2 & 537 0C. Steam entry to HP turbine through two combined main stop & control valves and to IP turbine through two combined reheat stop and control valves. Reheated steam pressure and temperature: 34.5 kg/cm2 & 537 0C. 210 MW KWU turbine is a tandem compounded, three cylinders, single reheat, condensing turbine provided entirely with reaction blading. Number of stages: HPT - 25 stages, IPT - double flow with 20 reaction stages per flow and LPT double flow with 8 stages per flow. Six steam extractions for feed /condensate water heating have been taken from HPT exhaust & 11th stages of IPT for high pressure heaters, from IPT exhaust for de-aerator and from 3 rd , 5th & 7th stages of LPT for low pressure heaters. The individual turbine rotors and the generator rotor are connected by rigid couplings. 250 MW (KWU) steam turbine (Mejia TPS U # 5&6):
HP turbine inlet steam: 147.10 kg/cm2 & 537 0C. Steam entry to HP turbine through two combined main stop & control valves and to IP turbine through two combined reheat stop and control valves. Reheated steam pressure and temperature: 34.95 kg/cm2 & 537 0C. 250 MW KWU turbine is a tandem compounded, three cylinders, single reheat, condensing turbine provided entirely with reaction blading. Number of stages: HPT - single flow with 25 stages, IPT – single flow with 17 stages and LPT double flow with 8 stages per flow. Six steam extractions for feed /condensate water heating have been taken from HPT exhaust & 11th stages of IPT for high pressure heaters, from IPT exhaust for de-aerator and from 3rd, 5th & 6 th stages of LPT for low pressure heaters. The individual turbine rotors and the generator rotor are connected by rigid couplings. HP rotor is supported by two bearings, a journal bearing at the front end of the turbine and a combined
journal and thrust bearing directly adjacent to the coupling with the IP rotor. The IP & LP rotors have a journal bearing each at the end of the shaft. 500 MW (KWU) steam turbine (Mejia Ph II TPSs , unit 7 & 8):
HP turbine inlet steam: 170 kg/cm2 & 535 0C. Steam entry to HP turbine through two combined stop and control valves and to IP turbine through four combined reheat stop and control valves. Reheated steam pressure and temperature: 34 kg/cm2 & 535 0C. 500 MW KWU turbine is a tandem compounded, three cylinders, single reheat condensing turbine provided entirely with reaction blading. Number of stages: HPT - 17 stages, IPT - double flow with 12 stages per flow and LPT - double flow with 6 stages per flow. Six steam extractions for feed /condensate water heating have been taken from HPT exhaust & 7th stages of IPT for high pressure heaters, from IPT exhaust for de-aerator and from 2 nd , 3rd & 5th stages of LPT for low pressure heaters. The turbine shaft layout is similar to that of 210 MW KWU design. TECHNICAL SPECIFICATION OF TURBINES Maker BHEL Type Reaction turbine Type of governing Throttling No of cylinders 3 Speed 3000 rpm Rated output 210 MW for 1,2,3,4 and 250 for 5,6 Steam pressure before emergency stop valve 150 kg per sq. cm Steam temp before emergency stop valve 535ᴼC Reheat temp 535ᴼC Turbine oil system
The turbine oil system provides supply of oil to the journal bearings, maintains the temperature of bearings at desired level and acts as operating medium for hydraulic governor and also as sealing medium for hydrogen cooled generator. In units 500 MW and above, the practice is to use fire resistant fluids in place of lubricating oil for governor system to eliminate the risk of fire during oil leakage. Turbine oil system is operated by the following pumps: Main Oil Pump (MOP):
This pump is mounted in the front bearing pedestal and connected with turbine rotor through gear coupling. It supplies total system oil requirement during turbine in operation. Its discharge pressure is 20 Kg/cm2 for LMW set and 8 Kg/cm2 for KWU set at normal turbine speed. The lubrication oil is supplied through two injectors arranged in series. Starting oil pump (SOP) or Auxiliary oil pump (AOP):
It is a multi stage centrifugal oil pump driven by A.C. electric motor. 1 x 100% duty Starting oil pump for LMW set or 2 x 100% duty Auxiliary oil pump for KWU set are provided for meeting the requirement of oil of the turbo set during start up or shut down i.e. when Main oil pump is not in operation. A.C. lub. oil pump (AC LOP):
This is a centrifugal pump driven by an A.C. electric motor and can meet requirement for turbine lubrication system when the turbo-generator is on Turning Gear or under flushing of standstill condition of the m/c. This pump automatically takes over under inter lock action whenever the oil pressure in lubrication system falls to 0.6 kg/cm2 (gauge). D.C. emergency oil pump (DC LOP or DC EOP):
This is a centrifugal pump driven by D.C. electric motor and automatically cuts in during A.C. supply failure or when lubrication oil pressure falls to 0.5 kg/cm2 (gauge). Jacking oil pump (JOP):
JOPs are positive displacement pumps that provide high oil pressure (around 120 bar) under strategic journals of LMW turbine at Bokaro ‘B’ and KWU turbines. The oil pressure lifts the shaft slightly. This ensures no metal contact between journal and bearing. This greatly reduces the static friction and bearings wear, also the starting torque of turning gear drive. JOP can be stopped after the lubricating oil film is established in bearings at the time of putting TG in turning gear, as in the case LMW set at Bokaro ‘B’. Turbine oil purifier (Centrifuge):
The centrifuge purifies turbine oil by removing water and entrained solid matter. The centrifuge consists of a bowl which rotates on a vertical axis in an outer casing. It draws oil from the turbine oil tank or impure oil tank through an oil pump. After removing any water and entrained solid matter, the clean oil is returned to the oil tank or pure oil tank. Sludge is removed from the disc of centrifuge by periodic cleaning. CONDENSER Condenser:
Condenser is a huge heat exchanger and is located at the exhaust of LP turbine. The steam after driving turbine is dumped into condenser for recycling. The dumped steam is cooled by circulating water flowing through the tubes of condenser. The cooling takes place where the steam comes into contact with condenser cold water tubes through which cooling water is circulated with the help of Circulating Water (CW) pumps. The steam is thus condensed into water and is taken into the system for reuse. The hot circulating water on absorption of heat in condenser is either discharged into disposal canal (as in Durgapur TPS unit no. 3 ) or taken to the top of cooling towers where it is allowed to fall under gravity for lowering its temperature for recycling. Generally about 8-10 oC temperature difference is observed between CW inlet and outlet of the condenser. Turbine auxiliaries: Condensate extraction pump (CEP):
It is generally multistage, vertical, centrifugal pump and takes suction from hotwell on few inches of suction submergence i.e. on minimum net positive suction head (NPSH). Generally, one pump is in service and one as standby. A vent line connected with hotwell equalizes vapour pressure. It transports hotwell condensate to de-aerator through low pressure heaters. Its discharge is also used for: Sealing the gland of valves operating under vacuum. Temperature control of LP bypass steam. Filling siphons of main ejectors and siphon of drain expander. Actuating the forced closing non-return valves of turbine extraction steam lines.
Operation of group protection device for bypassing high pressure heaters. For cooling steam dumped through steam throw off device. For preparation of phosphate, hydrazine and ammonia solution.
Low pressure (LP) heater:
LP heaters take extraction steam from low pressure stages of turbine. T ube material is made of admiralty brass. Drain from heaters is either cascaded by pressure difference o r by drip pump into the next LP heater. De-aerator:
In De-aerator, condensate water is allowed to fall by gravity through steam scrubber to drive out oxygen and other dissolved gases from condensate water. It also heats the incoming water and acts as a reservoir to serve emergency. Pegging steam pressure, for maintaining required NPSH at De-aerator water temperature, is also applied in the De-aerator. Boiler feed pump:
Boiler feed pump (BFP) is a multistage pump provided for pumping de-aerator outlet water to economizer through high pressure heaters. Generally, three pumps each of 50% of total capacity are provided. Booster Pump, driven by the main BFP motor, is sometimes provided before feed pp to maintain required NPSH even with lower de-aerator height. Operation of BFP below NPSH may cause severe damage of the pump due to cavitation or vapour bounding. High pressure (HP) heater:
Regenerative feed heaters improve the cycle efficiency substantially. The driving force for heat transfer in feed heaters is the LMTD (Log mean temperature difference) and TTD (terminal temperature difference) between heater outlet drip and feed water. Tube material is generally of stainless steel. HP heaters may be in service either individually or in group. H.P/ LP bypass system:
The HP/LP bypass system diverts the main steam (MS) before turbine stop valve (MSV) to the cold reheat (CRH) line and LP bypass system diverts the hot reheat (HRH) before intercepting valves (IV) to the condenser, after reducing and matching the pressure & temperature of the steam in both the cases. Feed water is used as cooling water for HP bypass and condensate is used as cooling water for LP bypass station. This system serves the following functions: To establish flow through SH and RH for raising boiler parameters during start up. To warm up the steam lines. To match steam condition with turbine metal for quick or hot start up and for reheater protection. To shorten the start up time of the unit. To minimize DM water consumption during start up. To dump steam from boiler into condenser during huge turbine load throw off. To perform Turbine Trip to House Load (TTHL) operation. COOLING TOWER
Cooling towers cool the warm water discharged from the condenser and feed the cooled water back to the condenser. They thus reduce the cooling water demand in the power plants. Wet cooling towers could be mechanically draught or natural draught. In M.T.P.S the cooling towers are I.D. type for units 1-6 and natural draught for units 7&8.
NATURAL DRAUGHT FOR UNIT 7 AND 8 CHIMNEY A chimney may be considered as a cylindrical hollow tower made of bricks or steel. In MTPS the
chimneys of eight units are made of bricks. Chimneys are used to release the exhaust gases (coming from the furnace of the boiler)high up in the atmosphere. So, the height of the chimneys are made high.
ELECTRICAL OPERATION IN BRIEF The electrical operation of a power plant comprises of generation, transmission and distribution of electrical energy. In a power station both distribution and transmission operation can take place. When power is sent from power station to all other power station in the grid, it is known as distribution of power. When power plant is drivin g power from other power station it is known as transmission of power/electrical energy. ELECTRIC GENERATOR In M.T.P.S. there are 6 electric generators for units 1 to 6. These are 3 phase turbo generators, 2 pole cylindrical rotor type synchronous machines which are directly coupled to the steam turbine. The generator consist of 2 parts mainly the stator and the rotor. Stator: The stator body is designed to withstand internal pressure of hydrogen-air mixture without any residual deformation. The stator core is built up of segmental punching of high permeability, low loss CRGOS steel and are in interleaved manner on spring core bars to reduce heating and eddy current loss. The stator winding has 3 phase double layer short corded bar type lap winding having 2 parallel paths. The winding bars are insulated with mica thermosetting insulation tape which consists of flexible mica foil, fully saturated with a
synthetic resin having excellent electrical properties. Water cooled terminal bushings are housed in the lower part of the stator on the slip ring side. Rotor: Rotor is of cylindrical type shaft and body forged in one piece from chromium nickel molybdenum and vanadium steel. Slots are machined on the outer surface to incorporate windings. Winding consists of coil made from hand drawn silver copper with bonded insulation. Generator casing is filled up with H2 gas with required pressure, purity of gas is always maintained>97%. Propeller type fans are mounted on either side of the rotor shaft for circulating the cooling gas inside the generators.
TURBOGENERATOR TRANSFORMERS The electricity thus produced by the generator then goes to the generating transformer where the voltage is increased for transmission of electricity with minimized copper losses. In general a transformer consists of primary and secondary windings which are insulated from each other by varnish. In M.T.P.S. all are either oil cooled or air cooled. Some of the transformer accessories are: 1. Conservator tank 2. Buccholz relay 3. Fans for cooling 4. Lightning arrestors 5. Transformer bushings 6. Breather and silica gel. AUXILIARY TRANSFORMRERS Station Service Transformers
Normal source to the station auxiliaries and standby source to the unit auxiliaries during start up and after tripping of the unit is station auxiliary transformer. Quantity of station service transformers and their capacity depends upon the unit sizes and nos. Each station supply transformer shall be one hundred percent standby of the other. Station service transformers
shall cater to the simultaneous load demand due to start up power requirements for the largest unit, power requirement for the station auxiliaries required for running the station and power requirement for the unit auxiliaries of a running unit in the event of outage of the unit source of supply. The no. and approximate capacity of the SST depending upon the no. and MW rating of the TG sets are indicated below.
Unit Auxiliary Transformer The normal source of HV Power to unit auxiliaries is unit auxiliary transformer. The sizing of the UAT is usually based on the total connected capacity of running unit auxiliaries i.e., excluding the stand by drives. It is safe and desirable to provide about 20% excess capacity than calculated. The no. and recommended MVA rating of unit auxiliary transformers are as shown in the above table: The UATs shall have Ddo(ungrounded system) or Dy1 (for grounded system) connection with on load tap changer to provide +10 % variation in steps of 1.25 %. Usual cooling arrangement to unit auxiliary transformers are ONAN. Radiators are usually divided in two equal halves. EXCITATION SYSTEM The purpose of excitation system is to continuously provide the appropriate amount of D.C. field current to the generator field winding. The excitation system is required to function reliably under the following conditions of the generator and the system to which it is connected. Functional components of an excitation system :
A good excitation system consists of properly co-ordinated functional components which are a) Excitation Power source b) Semiconductor Rectifier c) Voltage controller d) Protective, limiting and switching equipments e) Monitoring, Metering and indicating equipments and f) Cooling system Types of Excitation System :
In earlier days DC excitation system was in use. Increase in generator capacity in turn raised the demand of excitation power which was notachievable by the DC exciters. This l ed to the accelerated development of AC excitation system in pace with generator capacity. With the maturing of solid state semiconductor technology AC excitation system found to be superior technically as well as economically. Excitation system can be categorized and subdividedinto the following : a) D.C. excitation system
i) Pilot Main Exciter excitation system ii) Rotating Amplifier excitation system. b) A.C. excitation system
i) Rotating High Frequency excitation system ii) Static excitation system iii) Brushless excitation system.
CONCLUSION The vocational training had been concluded in a very efficient way. We have acquired through knowledge about generation, transmission and distribution of power. MTPS being one of the largest power station in eastern India, had been acting as a pioneer in power generation over a decade. MTPS is a part of DVC which governs the power generation for industrial and commercial requirement and attenuate the economic as well as social well-being of humankind. We have carried out the training under well experienced and highly qualified engineers of MTPS. The work culture of DVC is very noticeable and very energetic. Although this is an old power plant, the machines and entire instruments are functioning very well due to proper maintenance and skill in handling them. I was able to acquire practical knowledge of the industry and about some theoretical engineering studies. My heart is not permitting me to stop, but I have to stop due to time and space limitations.
