30 MW POWER PLANT

Power Station

ACB (India) Ltd.30MW Power Plant, Chakabura

An ISO 9001 Certified Company

FOREWORD

For development of technical competence for execution of any technical activity efficiently and safely, the individual handling/in charge of the activity must have to fulfill two important requirements. a) Up to dated adequate direct knowledge related to the concerned portion of the

activity. b) Required skill which can come only with training including on the job and

experience of dealing with the activity. The technical personnel too should have sufficient related knowledge of the other fields (other than his own field) of the activity without which the activity cannot be completed. Only with possession of the above, it becomes possible to achieve the desired results from the efficient performance of any activity and consequently achieve the commercial and financial results so that the ventures continue to remain viable and in conformity with the projected objectives of the venture. The above two are very essential to perform the activity not only competently but also in a safe manner. Unsafe operations will eventually lead to accidents and increased down time which will impinge on the commercial results of the venture. Accidents lead to increased down time, affecting the performance directly and contribute very negatively in an indirect manner by bringing down the morale of the team as some accidents cause injuries contributing to pain and human misery. This booklet is commendable effort in providing valuable information and knowledge relating to the important aspects of thermal power generation like Boilers, Steam Turbines, Turbo Generators, Water Treatment Plant and Coal Handing Plant which will be extremely useful to the personnel who are involved in the operation of the thermal power generating plants. It is very necessary that this booklet is made to reach the concerned personnel who must go through the same as well meeting session need to be arranged for interaction, clarification and also to get feed back to ascertain the extent to which the concerned personnel have imbibed the knowledge. I feel very happy that such booklet is being published and hope that such efforts shall continue on other aspects of various technologies.

Date : Tuesday, February 16, 2010 Ganesh Chandra Mrig Place : Gurgaon Managing Director

FOREWORDFOREWORD

I had spent some 35 years in Power Plant O&M before coming to Chakabura Plant. During that period I had seen Boilers and Turbines of different Manufacturers such as Franco Toshi Italy, CEM France, Babcock & Wilcox; ABL, BHEL, Polish and so on. It taught me that names and labels may change but the vkRek

1. Introduction 12. A TPS at a Glance 33. Steam generator

3.1 Coal 83.2 Air 113.3 Combustion System 12

3.3.1 Fluidization system 133.4 Details of Boiler Plessure Parts 153.5 Fans 263.6 Electro Static Precipitator 323.7 Ash handling system 41

4. Turbine 434.1 Governing system 464.2 Gland sealing system 494.3 Air evacuation system 504.4 Rankine cycle 504.5 Regenerative heating system 524.6 Condenser 544.7 Cooling tower 554.8 Vibration Monitoring System 58

5. Turbo Generator 605.1 The operation of generator on infinite bus-bar 655.2 Circuit breaker 715.3 Transformers 775.4 Motors 875.5 Variable Frequency Drive 885.6 UPS 93

6. Water Treatment Plant 1036.1 Pre treatment plant 1046.2 Reverse Osmosis System 1066.3 Water chemistry

6.3.1 Boiler water chemistry 1096.3.2 Cooling water chemistry 114

7. Control & Instrumentation7.1 DCS 118

8. Lubricating oil & additives 1399. Plant Performance History

9.1 Monthly Gen, Exp. PLF & A.F. 1409.2 Details of Unit trippings/outage period 1429.3 Problems, modifications and achievments 145

...... INDEX ......

Sr.No Chapter Title PageNo

1Importance and need of Electricity cannot be overemphasized in a modern world. It is the most convenient form of energy because: i) Travels at speed of light (only slower than the human mind!) ii) Transmission is without pollution. iii) Can be easily converted to other forms such as Heat, Light, and Mechanical.

Since energy can neither be created nor destroyed; Generation of Electrical Energy (Power) means conversion of other form to Electrical Thus we have Wind Power, Tidal Power, Solar Power, Hydro Electric Power and Thermal Power (Which includes Nuclear Power). Of all these, Coal based Thermal Power is the major player with @75% share.

Our country with an estimated 90 Gigatons of Bituminous/Sub-bituminous and 2 Gigatons of Anthracite (1 Gigaton= 1000 milliontons) has @ 10 % of the (recoverable) Total World Reserves of Coal. Needless to say that we will harness coal based thermal power for time to come. However the quality of this coal is inferior in terms of its Gross Calorific Value and Ash content. The ash content varies from 30 to 60 %. This means that substantial component of coal mined is bad as a fuel. If consumed by Thermal Power Plants, as mined, such coal will mean 1. Higher Transport Cost. 2. Higher Specific Coal Consumption.3. Higher expenditure on Ash management.4. Poor Loadability of Generating Units.5. Higher wear and Tear of Power Plant equipments and Pressure Parts. 6. More Auxiliary power Consumption.

MOEF (Ministry of Environment & Forests) Government of India, therefore, issued directives to the Thermal Power Producers to use coal with a maximum ash content of 34%. Large number of Coal beneficiation (or Washery) plants came into being as a natural consequence.

ACB (India) Ltd. the pioneer in coal beneficiation is the largest in India with installed capacity of over 30 MMT per annum!!

Beneficiation Plants use Density Differential between Coal and extraneous (non-coal) impurity by Float & Sink method. Different equipments are available to separate

1. IntroductionIntroductionWindow to Power Station

2Window to Power Station

Coal and Impurities. These include Rotating Barrel, Jigs (Batak, Baume etc.) using Heavy Medium such as Magnetite or Water with compressed air for pulsation so as to stratify a given mass of coal as per density/quality. Selection of a particular type of Washery/Benefication Equipment is done after conducting wash ability tests and plotting the results on a curve or graph showing the results of a series of float-and-sink tests. A number of these curves are drawn to illustrate different conditions or variables, usually on the same axes, thus presenting the information on one sheet of paper. Wash ability curves are essential when designing a new coal or mineral washery. There are four main types of wash ability curves: characteristic ash curve, cumulative float curve, cumulative sink curve, and densimetric or specific gravity curve.

Generally speaking 20% of coal washed is rejected not suitable as fuel in a conventional Boiler. But there is still combustible matter in this Reject. This is where a Fluidized Bed Combustor or Boiler plays its role. If we consider the fact that our country consumes 400 million Tons of coal per year we will generate 80 million tons of Reject if all of this was washed. This is adequate to feed a 6000 MW Power Plant with fluidized Bed Boilers. Thus there is a large potential for growth of such Generating stations.

3A Thermal Power Plant uses 5 main devices to generate electrical energy.1. Boiler or Steam Generator to convert chemical energy in the fossil fuel into heat of

combustion and transfer this heat to water. And generate dry steam of desired pressure, temperature and degree of superheat.

2. Steam Turbine to convert heat energy of steam to mechanical energy of rotation to drive the Generator.

3. Condenser to reject heat from the steam exhausted from Turbine after doing work. And to recover water for recycling.

4. Water Pump to feed high-pressure water to the Boiler at the expense of a fraction of energy generated (@ 2.4%).

5. A Turbo-Generator to convert mechanical energy into electrical using Turbine as Prime Mover.

It has been indicated elsewhere that beneficiation of coal generates substantial rejects which cannot be utilized in a conventional PF boiler. Fluidized Bed boilers can burn this coal-reject with particle size 4-6 mm effectively without pulverizing. These boilers have a bed made out of crushed refractory material. Particle size of the bed-material is 0.8

3 2.4 mm, bulk density @ 1100Kg/m and thermal conductivity of 0.24 cal/gm. Air nozzles below the bed are used to fluidize it. The Furnace is divided in 8 compartments with 20,000 Kg. bed material spread over. For initial light-up Charcoal or Firewood chips are laid on top of the bed in FIRST compartment and ignited with a torch. 4 Oil Burners located at 250mm elevation are also fired from the front side. On fluidization the whole mass of bed material in Compartment 1 becomes hot and reaches a temperature of 900-1000 C Whence (reject) coal firing is started. When Coal Flame is self sustainable oil and Charcoal firing is stopped. As hot bed material spills over to adjoining Compartments the temperatures there also rise sufficiently to accept Coal Feed. This is briefly the process of Boiler Light-up. Details are omitted purposely. Details of dimensions, numbers, cross-section etc. are given separately.

2.1 Boiler or Steam Generator

2.A Thermal Power Station at a GlanceA Thermal Power Station at a GlanceWindow to Power Station


A steam turbine is an engine in which heat energy of the steam is transformed into work. First, the steam expands through nozzle(s) and heat is converted into kinetic energy. Then, that kinetic energy is converted into work for spinning turbine blades. The usual turbine has four main parts. The rotor is the rotating part which carries the blades or buckets. The stator consists of a cylinder and casing within which the rotor turns. The turbine has a base frame on which the casing is mounted, and finally there are nozzles or flow passages which expand the steam. The cylinder, casing, and frame are often combined. Other parts necessary for proper operation would include a control system,a piping, a lubrication system, and a separate condenser.

There are many different types of turbines. The basic classification beinga) Impulse

(i) Simple or single-stage.(ii) Velocity-stage, Curtis.(iii) Pressure stage, Rateau.(iv) Combination pressure- and velocity-stage.

(b) Reaction, Parsons.(c) Combination impulse and reaction.

Most power plant turbines have One Impulse Stage followed by Reaction stages. Our 30MW Turbine has 15 stages (1 impulse wheel (stage) + 14 reaction stages). Bleeds are provided after 5th Stage (HP Heater), 10th Stage (De-aerator) and 13th Stage (LP Heater) for regenerative feed water heating.

2) Labyrinth Seals - the Turbine casing receives High Pressure (84Kg/cm steam at one end and discharges to condenser at a pressure much below atmosphere

2(0.1 Kg/cm ). Hence both ends have to be sealed to avoid leakage of steam outside and ingress of air inside. In addition there is differential pressure across each stage of Fixed Nozzles (Diaphragms) in turbine which need sealing to prevent steam bypassing the moving blades. This is achieved by providing Labyrinth Gland Seals. These are serrations on the Turbine Shaft and matching grooves on the stator casing so as to provide a high resistance path for leakage steam. The actual arrangement is very complex with stepped/double-stepped/vernier type labyrinths for glands and Axial-Radial labyrinths for inter-stage seals etc. With Turbine shaft speeds of 7059 rpm the labyrinth clearances are also critical (@ 0.5mm).

Working principle of Generator - The working principle of the Alternator OR Generator is very simple. When a conductor is placed in a changing magnetic flux an emf is induced proportional to the rate of change of flux.

2.2 Steam Turbine

2.3 Turbo Generator


The Generator Rotor is an electromagnet which provides magnetic flux in the air-gap ofA the Alternator and the rotation of Turbine provides a constant variation (Sinusoidal).. The conductor is in the form of Coils or Winding on the Stator (armature).

The situation complicates when a LOAD is connected to stator. The flow of load-current through the Armature (Stator winding) gives rise to armature-reaction which opposes the change of flux by

a) Cancelling the Flux demagnetising. b) Braking the Generator Rotor- slowing down the speed.To nullify the effect of armature reaction an AVR (automatic voltage regulator) is

introduced which controls the field-excitation-current to maintain the terminal voltage constant. AVR is a critical device and is responsible for safe operation of Generator within the Capacity Curve at all times.

An Operation Engineer must have conceptual understanding of these requirements and should keep a watch on electrical parameters. He/She should know that Overvoltage and Under-voltage Trip is set at 10% of 11KV. In case 132KV Grid voltage is HIGHER we have to shift the GT Tap to LOWER position. A close look at the OLTC panel will clarify why?

A) Raw Water and Pre-treatmentWe have constructed an Anicut at Korai village on the confluance of Salia and

3Kholar Nallahs. The dam has a storage capacity of @ 200,000 M . The present dam height is 2 m and 8 nos of auto-opening flood-gates are provided. These gates open without external power during floods and close automatically when water recedes. They are hydraulically damped to prevent crash-closing during monsoon floods.

An intake-well is constructed upstream of dam on Salia Nalla for drawl of raw water with the help of (2x 100 TPH + 2x250 TPH ) 4 Pumps and a 400mm size MS-ERW pipeline. Normally 2 pumps are in service and bring water to the water pre treatment plant. This comprises :-

1. A settling cum clarifier tank. Here chemicals are dosed for clarifloculation so as to bring down the turbidity.

2. RGF- or rapid gravity filter where water from settling tank passes through a graded sand filter to arrest suspended particals left over. The output water to

3a storage tank of 16000 M . This tank is the reservoir for CW make up and input to RO/DM treatment plant

2.4 Water Treatment Plant


Power plant requires 2 different qualities of watera. For Boiler -The boiler water has to be free from all dissolved salts to avoid

deposits on tubes. It must also be free from oxygen which causes corrosion. This is achieved by Demineralization and De-aeration. RO i.e. Reverse Osmosis process is used to remove all dissolved salts from water. The product of RO plant is passed through a Mixed Bed of Anion & Cation to yield high quality DM water.Such DM water is highly aggressive under high temperature conditions obtained in boiler and therefore it is maintained in alkaline regime between 9.8 to 10.2 pH. TSP (tri-sodium phosphate) is dosed to remove hardness and to boost pH in the boiler Drum. Feed Water is de-aerated before going to boiler and to scavange any oxygen traces left Hydrazine is dosed in the suction line of BFP. O in water is brought down to < 7parts per billion. 2

b. Circulating Water CW:- Old power-plants used Softened Water with hardness less than 40. This was when Condenser Tubes were of Brass/Admiralty- Brass. This required a Zeaolite Softening plant. Water requirement was much higher. Now Condenser Tubes are of SS which do not corrode and clear water is adequate as CW. To optimize water requirement COC (Cycles of concentration) are increased to 7-8. The philosophy followed is to keep the solids in solution and not to allow them to form scale on Condenser Tubes. This is achieved by dosing a) Scale Inhibitors- orthophosphate & phophonates b) De-scaling agent- Sulphuric Acid. c) Micro biocides to kill algae and bacteria.

The function of CHP is to supply required quantity and quality of coal to the boiler. It receives coal from washery through a conveyor belt, screens it with a vibrating screen and sends -6mm size to the boiler bunker. Oversize coal is sent to the ring-granulator for crushing to -6mm. There is provision to receive coal through trucks. Such coal is fed through a ground hopper and then to the vibrating screen. Since the required (size & quantity) of coal is available as received the CHP is mostly limited to conveying.

2.5 Coal Handling Plant.


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83. Steam GeneratorSteam GeneratorWindow to Power Station

3.1 COALCoal formation is the outcome of a slow geological process taking millions of years.

Under suitable conditions dead biotic matter (such as trees) get successively transformed in Peat, Jet, Lignite, Sub-bituminous, Bituminous coal , Anthracite, Graphite etc .Of these members of Coal family thermal power plants utilize Lignite, Sub-bituminous & Bituminous varieties only.

World coal reserves: At the end of 2006 the recoverable coal reserves amounted to 800 - 900 Gigatons (1 Gigaton=1000 Million Tons. At the present consumption rate, this would last 264 years. The largest reserves are found in the USA, Russia, Australia, China, India and South Africa.

Proved recoverable coal reserves at end-2006 (in million tonnes)


Recent discoveries in the region of Pakistan have given rise to a discovery of nearly 185 billion tonnes. Coal Beneficiation or Coal Washing :- The Coal Deposits in our country are huge in terms of quantity. However the quality of this coal is inferior in terms of its Gross Calorific Value and Ash content. The ash content varies from 20 to 60 %. Thus substantial portion of our coal reserve is difficult to use as a fuel. If consumed by Thermal Power Plants as mined this will mean.1. Higher Specific Coal Consumption. 2. Higher Transport Cost. 3. Higher expenditure on Ash management.4. Poor Loadability of Generating Units. 5. Higher wear and Tear of Power Plant equipments and Pressure Parts.

MOEF (Ministry of Environment & Forest) Government of India therefore issued directives to the Thermal Power Producers to use coal with a maximum ash content of 34%. Large number of Coal beneficiation (or Washery) plants came into being as a natural consequence.

ACB India Ltd. is a pioneer in coal beneficiation and the largest with an installed capacity of over 30 MMT per annum!!

Coal Beneficiation Plant works on principle of Density Differential between Coal and extraneous (non-coal) impurity by Float & Sink method. Different equipments are available to separate Coal and Impurities. These include Rotating Barrel, Jigs (Batak, Baume etc.) using Heavy Medium such as Magnetite or Water with compressed air for pulsation so as to stratify coal as per density/quality. Selection of a particular type of Washery/Beneficiation Equipment is done after plotting results of several float & sink tests on a curve or graph. A number of these curves are drawn to illustrate different conditions or variables, usually on the same axes, thus presenting the information on one sheet of paper. Washability curves are essential when designing a new coal or mineral washery. There are four main types of washability curves: characteristic ash curve, cumulative float curve, cumulative sink curve and densimetric or specific gravity curve Elements in Coal

There are 3 elements in coal that combine with Oxygen in the air during the combustion process. The main and important element is Carbon which could be around 20%.Then there is Hydrogen in the range of 1 % to 2 % and Sulphur in the range of 0.3 % to 0.8 % . Apart from this there is Volatile matter, Hydrogen, Nitrogen, Oxygen and Ash. Stoichiometric or theoretical air quantity.This is calculated based on the chemical reaction between the elements and oxygen.1. Carbon combines with Oxygen to form Carbon-dioxide and heat.

C+ O > CO 1 C +32/12 O > 44/12 CO 1 kg2 2 2Carbon +2.67 kg Oxygen > 3.67 kg

2. Hydrogen Combines with Oxygen to form Water and heat2H + O > 2H O 1 H +32/4 O >36/4 H O 1 kg Hydrogen22 22+8 kg Oxygen >9 kg Water

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Heat of combustion (GCV) table

Window to Power Station

3. Sulfur- S + O > SO 1 S +32 /32 O >64 /32 SO 1 kg Sulfur2 2 2+1 kg Oxygen >2 kg Sulfur Dioxide.

Heating valueThe heating value or calorific value is a characteristic for each substance, and is the

amount of heat energy released during the combustion of unit mass of the substance,in: kcal/kg, kJ/kg etc. Heating value is commonly determined by use of a bomb calorimeter.

Higher Heating Value (HHV) (or Gross Calorific Value) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced. This is the same as the thermodynamic heat of combustion since the enthalpy change for the reaction assumes a common temperature of the compounds before and after combustion, in which case the water produced by combustion is liquid.

Lower Heating Value (LHV) (or net calorific value) is determined by subtracting the heat of vaporization of the water vapor from the higher heating value. This treats any H O formed as a 2vapor. The energy required to vaporize the water therefore is not realized as heat.

Gross heating value (see ARB) accounts for water in the exhaust leaving as vapor, and includes liquid water in the fuel prior to combustion. This value is important for fuels like wood or coal, which will usually contain some amount of water prior to burning.

ARB (As Received Basis) indicates that the fuel heating value has been measured with all moisture and ash forming minerals present.

ADB (Moisture Free) or Dry indicates that the fuel heating value has been measured after the fuel has been dried of all inherent moisture but still retaining its ash forming minerals.

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Temperature Density Specific heat Thermal capacity conductivity

- t - - - - c - - l -p3(C) (kg/m ) (kJ/kg K) (W/m K)

0 1.293 1.005 0.0243

40 1.127 1.005 0.0271

80 1.000 1.009 0.0299

120 0.898 1.013 0.0328

140 0.854 1.013 0.0343


Ratio compared to MolecularDry Air(%) Mass ChemicalGas-M- SymbolBy By (kg/kmol)Volume Weight

Oxygen 20.95 23.20 32.00 O2Nitrogen 78.09 75.47 28.02 N2CarbonDioxide 0.03 0.046 44.01 CO2

Hydrogen 0.00005 ~ 0 2.02 H2Argon 0.933 1.28 39.94 ArNeon 0.0018 0.0012 20.18 Ne

Helium 0.0005 0.00007 4.00 HeKrypton 0.0001 0.0003 83.8 KrXenon 9 10-6 0.00004 131.29 Xe

3.2 AIRCommon properties for air are indicated in the table below Specific Heat Cp of Air is 1 to

1.01 kJ/kg C. Its Thermal Conductivity increases from 0.024 to 0.034 W/mC with temp. rise from 0C to 140C

Constituents of AirAir is a mixture of gases - 78% nitrogen and 21% oxygen (by volume) with traces of water

vapor, carbon dioxide, argon, and various other components. Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.

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Other components in air in parts per million (ppm).1. Sulfur dioxide - SO - 1.0. 2. Methane - CH - 2.0.423. Nitrous oxide - N O - 0.5 4. Ozone O - 0 to 0.07.325. Nitrogen dioxide - NO - 0.02. 6. Iodine - I - 0.01 2 27. Carbon monoxide - CO - 0 to traces. 8. Ammonia - NH - 0 to traces3

The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature .The composition of air is unchanged until elevation of approximately 10,000 m. The average air temperature diminishes at the rate of 0.6C for each 100 m vertical height "One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a

2760 mm column of mercury at 0C sea level and at standard gravity (9.81m/sec )Air contains 23.2 % by weight of Oxygen. The theoretical Air required to burn the coal is calculated accordingly..

The Heating value of Coal also depends on the elemental Carbon and Hydrogen. This means that the air required and the heating value have an almost fixed relationship. The theoretical air required for a unit heating value is practically a fixed value. This is around 0.332 kg of air for one MJ(1MJ= 239 Kcal) of heat input. This is true for a wide range of coals used in power plants.

As a power engineer it is sufficient to remember that for our unit we should have 4kg air per kg of coal. It should also be borne in mind that composition of air does not change. However its

3 3density reduces from1.2kg/m (20C) to 0.8kg/m (160C) as temperature rises.

Combustion reaction: Combustion is the high temperature oxidation of the combustible elements of a fuel with heat release. Combustion is the chemical reaction which takes place when combustible element (Carbon in coal) combines with the oxygen in air, and in so doing gives off large quantity of heat.The basic chemical equation for complete combustion are

C+O ------ CO + HEAT2 21(g) + 2.67 (g) -------3.67(g) + 33.94 kjFrom above equation 1gram of Carbon requires 2.67 grams of oxygen (11.50 gram of air)

for complete combustion and will produce 3.67 gram of Carbon dioxide and release 33.94 kj of heat.When insufficient oxygen is present, the carbon will be burned incompletely with the formation of carbon monoxide i.e 2C+O ------2CO.21. In order to burn a fuel completely, four basic conditions must be fulfilled.2. Supply enough air for complete combustion of fuel.3. Secure enough turbulence for thorough mixing of fuel and air.4. Maintain a furnace temperature high enough to ignite the incoming fuel air mixture.5. Provide a furnace volume large enough to allow time for combustion to be completed.

3.3 COMBUSTION SYSTEM

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3.3.1. FLUIDISED BED COMBUSTIONFluidized bed: When air or gas is passed through an inert bed of solid particles such as crushed refractory material or sand on a grid, the air initially will seek a path of least resistance and pass upward. With further increase in the velocity, the air bubbles through the bed and particles attain a state of high turbulence. Under such conditions, the bed assumes the appearance of a fluid. Hence the name is 'Fluidized Bed'.

Mechanism of Fluidized Bed Combustion: If the bed material in a fluidized state and is heated to ignite the temperature of the fuel and the fuel is injected continuously into the bed, fuel will burn rapidly and the bed attains a uniform temperature due to effective mixing. This is called fluidised bed combustion.

While it is essential that temperature of bed should be at least equal to ignition temperature oof and it should never be allowed to approach ash fusion temperature (1150 C) to avoid melting of

ash. This is achieved by extracting heat from the bed by conductive and convective heat transfer through tubes immersed in the bed.

If air velocity is too low, fluidization will not occur and if the gas velocity becomes too high , the particles will be entrained in the gas stream and lost. Hence to sustain the stable operation of the bed , it must be ensured that the air velocity is maintained between minimum fluidisation velocity and particle entrainment velocity.Advantages of FBC boiler:1. It is designed to burn low grade coals like washery rejects.2. Low combustion temperature of the order of 800C 900C facilitates burning of coal with

low ash fusion temperature, prevents NOx formation3. High sulfur coals can be burnt efficiently without much generation of SOx by feeding lime

stone.4. High turbulence of bed facilitates quick start-up and shut down. Departure from nucleate boiling (DNB)

The point at which the heat transfer from a fuel rod rapidly decreases due to the insulating effect of a steam blanket that forms on the rod surface when the temperature continues to increase.Boiling:- Water is boiled in the Steam Generator to form steam. However the process is not simple and it is necessary to understand various stages or phases involved. As an operation engineer it is sufficient to know that heat transfer between the Heating Surface and the Heat Recipient (water/steam) depends primarily on the temperature difference. There are 3 main regimes of thermodynamic heat transfer namely Natural Convection, Nucleate and Film Boiling. If we look at the Pool Boiling Curve with T on Abscissa and Heat (transfer) Flux on the Ordinate we get a plot as shown below.

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Pool Boiling (Nukiyama) Curve

Heater temperature

Heat

su

pply

AB-NUCLEATEBOILING BC-FILM BOILING

OA-CONVECTION

The segment O-A is the initial heating as per Convection Mode. The temperature rises smoothly with no bubble formation. This is when T is up to 8-10 C. The portion A-B shows a high rate of heat transfer and reaches a peak value at B. This is when local heat is sufficient to activate nucleation sites on the heating surface and vapor bubbles are formed. Very rapid bubble formation causes strong velocities within the liquid film and heat transfer increases. At B the heat flux reaches its maxima T @ 100C. This is called the Critical Temperature Difference Point or DNB (departure from nucleate boiling) or Burn-out point. Several physical and thermodynamic phenomena are occurring as we approach B. The numerous nuclei and rapid evolution of bubbles prevent the liquid particles reaching the heating surface which is starved. The heat flux starts reducing from B and reaches minima at C. A further increase in T increases the heat flux again. This is where Film Boiling starts. Here a continuous layer of vapor covers the heating surface retarding heat flux. This layer thickens as T is increased. But a further increase in Temperature (T @ 500C) increases heat transfer due to Radiation and perhaps convection within the vapor film.

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Window to Power Station3.4 Details & Specification of Boiler Pressure Parts

2125 TPH, 515C, 87kg/cmFurnace DataBed Size :- 10.13 x 6.34 Mtr.Expanded Bed Height - 1.06 MtrHeat I/P --: 99675486 Kcal / Hr Or 416643.5 MJ/ Hr

2Heating Surface Area = 5664.5 M 3 Boiler Volume = 80.3 M

2LHS Safety Valve set Pr 102 kg/ cm2RHS Safety Valve Set Pr 103 kg/ cm2M.S.L - 91 Kg /cm

Natural Circulation Circuit :-It Consists of pressure parts Circuit , Beginning from Down-Comers to the Steam Drum Via Risers . Natural Circulation occurs betn the water in Down Comers

&Water steam mixture in water walls.The Hydrostatic Head- is the difference at the feet of the down comers and the water walls

is equal to the sum of pressure drops due to the friction & acceleration of flow medium in the circuit . circulation will be higher at the lower pressures & at low loads.

Circulation will be reduced when feed water temp increases and when drum level is low. The Steam & Water System -:- It Consists mainly of Economiser, Steam

drum,Evaporator ,Bed Evaporator, Superheater ( CSH, RSH, BSH) & Desuper heater( Primary, Secondary).

Parts of Evaporator :- Steam Drum -01 Down Comers 06 I/L Bottom Ring Headers 04 Bed Coils (240 coils ) Bed Coils headers (04 nos) Water Walls ( 15 panels) Roof Headers ( 08 nos) Risers ( 14 nos) Parts of super heater :- Convection super Heater 01 Radiant Super heater 01 Bed Super heater 01

DE- SUPER HEATER :- Primary De superheater-01 Secondary De superheater-01

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Window to Power StationPrimary De superheater Located Betn CSH O/L & RSH I/L Secondary De superheater- Located Betn RSH O/L & BSH I/L

Bed Evaporator Bed Coil Tube Types Outer coil 156 nos Inner coil 84 nos Total Coils 240 nos Tube size - D 51 x 6.35 thk ( Both types) Tube Material SA 210 Gr A1 ( For Both types)

Convection superheater Coils No of coils 65 ( 65 nos of loops) No of banks 01 Type of flow Cross Coil Type - Pendant type Tube size D 44.5 x 4.5 mm Tube material SA 210 Gr A1 Longitudinal pitch -130 mm Radiant Superheater Coils No of coils 60 ( 12 loops each contains 5 coils , 12 x 5) No of banks 01 Type of flow Cross Coil Type - Platen type ( Rectangular) Tube size D 44.5 x 4.5 mm Tube material SA 213 T11, SA 213 T22, SA 213 T91 Longitudinal pitch -89 mm

Bed Superheater Coils Bed Coil Tube Types Outer coil 36 nos Inner coil 36 nos Total Coils 72 nos Tube size - D 44.5 x 5thk ( Both types) Tube Material SA 213 T 91 ( For Both coils) Coil Type - U-BendWater Wall Type Membrane type (with fins) Tube size - D 51 x 4.5thk Tube Material carbon steel , SA 210Gr A1

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8.6 Mtr

1.37 Mtr

9.6 Mtr

shellthk= 90MM

CAP - 125 TPH

PR - 87 KG/CM MAX SuperHeated Temp- 5155c

Saturated Temp - 313c

Hydro Test Pr - 154 Kg/cm

BOILER MAKE :- CetharVessels Pvt LtdFUEL :-Washary RejectsTYPE :-AFBC,SingleDrum,WaterTubeBoiler

BOILER ACESSORIES :- ECONOMISER,APH,SUPERHEATER

BOILER MOUNTINGS & FITTINNGS :-

1) Bi-colorWater level guages :- 02nos

2) Remote Electronic Level Gauge- 02 nos3) Ram Bottoms Safety Valve- 03 nos4) Local pressure Guage - 01 nos5) N\U+2082 Capping/ Aux stub - 016) Drum Vent - 017) Steam O/L stubs- 048) Down comers - 069) Risers - 14 nos10) Feed Pipe - 0111) Steam seperators - 26 nos12) Screen Box - 24 Nos13) Drain pipes - 07 Nos14) CBD Connections - 0115) IBD Connections - 01

STEAM DRUMSTEAM DRUM

Mat= SA 516 GR 70

Down Comer Quantity 06 Nos Pipe size D 273.1 x 15.09 thk (4 nos) , D 219 x 12.7 Thk Pipe Material SA 106Gr B

2

2

18

AIR PREHEATER SPECIFICATIONS Type - Recuperative Type (Tubular) Air Flow--- Through tubes Gas Flow -- Over Tubes No of passes-- 03 By Pass--Bet n Pass-I & Pass-II Ducting Material- M.S. Total Air Heater Tubes4680 Nos Each Pass 1560 Tubes Tube Material M.S (Except Top 325 Tubes) Tube Size 60.3 x 2.34 thk (m.s), Length= 4mts Tube Sheet 25mm Thk, Hole Dia = 61.3 Grade -- ASTM-A423GR I/ Corten steel.(Ist pass 325tubes) Air I/L Temp Ambient Hot Air Temp155C

APH:- Aph is a accessory of boiler . In airpreheater , air used for combustion of fuel is being preheated & supplied to the furnace by means of forced draught fans. The air at ambient temp is drawn from atmosphere & heated to the temp of 155C then used for conveying & combustion of coal. The sensible heat from the flue gases left after economiser is recovered in aph to increase the combustion efficiency & thermal efficiency of boiler. The hot flue gases at 250 270C enters in APH and the ambient air flowing through the tubes picks up this sensible heat by heat transfer & gains the temp upto 155C. The internal construction drawing is attached.


DRUM INTERNALS

RISER

RISER

DOWN COMER

TURBO SEPERATOR

SCREEN DRIER

FEED PIPE

DRAIN PIPE

BAFFLE

STEAM OUTLET

VORTEX

SPINNER

19

ECONOMISERspecifications Tube Size = 51Dia x 4.5thk Tube Material = SA 210Gr A1

2 Heating surface Area = 2989m

3 Water Holding Capacity = 25.95m Eco coil Type I = 24 Nos ( Bank no-1) Eco coil Type II= 24 Nos (Bank no-2) Total Coils = 48 Nos Feed water I/L Temp = 200C Feed water O/L Temp = 295CEconomiser :- It is a accessory of boiler. where feed water for evaporator is preheated & pumped to the steam drum of boiler. The feed water enters at about 200C & leaves the economizer at 295- 300C. In economizer,the sensible heat from the flue gases at 450- 460C is get transfered to the feed water flowing through the coils gains the temp upto 300C.Economiser increase the thermal efficiency of boiler. The internal construction drawing is attached.


Flue Gases O/L

I/L3940 x 5379

ASHHOPPER

AIR PREHEATER

PASS-1

PASS-2

PASS-3

1560 Tubes M.S

325 Tubes Cortensteel

1560 Tubes

1560 Tubes

Hot AirO/L

Flue GAS

Cold AIR I/L

Aerofoil

BY PASSDAMPER

Tubesheet=25mmthk

20


6666

9250

12@

153

335

12@

153

1200

12@

153

335

12@

153

=18

36

ECONOMISERECO I/LHEADER

ECO O/LHEADER

BANK II

Bank ITotal Coils = 48 NosBank I = 24 CoilsBank II = 24 CoilsTube Size = 51 x 4.5 thkTube Material = SA210GrA1

21


CSH COIL

30

30200

57

GOOSE NECK

1580

4212

3291

5969.5

BURNER OPENING

RSH COIL

WATER WALL

RSH O/L HDR

RSH O/L LINK

RSH I/L HDR

Roof Center HDR75

75

900

1044.5De-superheater-1

CSH I/L

CSH I/L HEADERCSH O/LHDR

CONNECTING LINK

BOILER FURNACE

10.13mx6.34mBED SIZE

BOTTOM RING HEADER

EXPANSION BELLOW

TOP HEADER

22


360x

300

360x

300

240

240 1515

105

105

3.15

3.15

530

530

8888 157

157

220

220

330

330

3535

2020

104

104

190

190

11.9

11.9

gap

gap

123

123

3535

2525 1010

516

516

25.0

25.0

4040

THREAD LG THREAD LG

SECO

NDAR

Y SE

PERA

TOR

QUAN

TITY-

26 N

osSI

ZE- 2

20MM

STEA

M SE

PERA

TOR

DRUM

INTE

RNAL

S

PRIM

ARY

SEPE

RATO

R

SPIN

NER

HOOK

ROD

FEED

PI

PE

5050

PIPE

168

.3x7.1

1 thk

EACH

55 no

s

30 H

oles A

T 135

& 18

062

10

END

PLAT

E6M

MTHK

ECO

O/L P

IPE

DRAI

N PI

PE

RODW

ATER

TRAY

UNIO

N(FE

MALE

)

SCRE

EN B

OX QUAN

TITY

DRAI

N PI

PE=7

nos

Scre

en =

24 no

s

23


52%%

D9

1

132

1580

30

123

8

30

213

1325

2

12

3

1580

153

82

143

BO

TT

OM

H

EA

DE

R

TO

TA

L L

EN

GT

H -6

78

6

BE

D C

OIL

T

YP

E-I

BE

D C

OIL

T

YP

E-II

TO

TA

L L

EN

GT

H - 5

41

8

2270

TOP

H

EA

DE

R

106

@ 20

=

21

202

86

16

4

164

286

3027

3027

150

@ 20

=

3000

Ty

pe

-1

= 1

56

c

oils

Ty

pe

-2

=

8

4 c

oils

To

tal N

os

=

2

40

c

oils

Tu

be

S

ize

=

D

5

1 x

6

.3

5 m

m th

kTu

be

m

ate

ria

l =

S

A2

10

GR

A1

24


MIXI

NG NO

ZZLE

OU

TER

PIPE

PART

NO

1

23

41

176 1

55 95

90 7

5

INNE

R PA

RTS

OF MI

XING

NO

ZZLE

MIXI

NG NO

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CTIO

N VI

EW

ASSE

MBLY

OF

IN

NER

PART

S

HOT

AIR

COAL

COAL

+

AIR

25


26

DIFFERENCES BETWEEN FANS,BLOWER AND COMPRESSOR

Equipment Specific Ratio Pr. rise mmwcFans Upto 1.11 Upto 1136.Blowers 1.11 to 1.20 1136 to 2066Comprs Above 1.2 -


TYPICAL FAN PEAK EFFICIENCIESTYPE PEAK h %

CENTRIFUGALAirfoil,Backward Curved & Inc. 79-83Modified Radial 72-79Radial 69-75Pressure Blower 58-68Forward Curved 60-65

AXIAL FANSVane Axial 78-85Tube Axial 67-72Propeller 45-50

3.5 FANSIntroduction :- Fans provide air for ventilation and carry away flue gas. Fans generate a pressure to move air (or gases) against a resistance caused by ducts, dampers, or other components in a fan system. The fan rotor receives energy from a motor shaft and transmits it to the air/gas.

Fans, blowers and compressors are differentiated by the method used to move the air, and by the system pressure they must operate against. Fan Types

Fan selection depends on the volume flow rate, pressure, type of material handled, space limitations, and efficiency. Fan efficiencies differ from design to design and also by types. Typical ranges of fan efficiencies are given in table below.

Specific Ratio= Outlet Pressure / Inlet PressureFans fall into two general categories: centrifugal flow and axial flow.In centrifugal flow, airflow changes direction twice - once when entering and second when leaving (Forward curved, backward curved or inclined, radial).

TYPES OF CENTRIFUGAL FAN BLADES

27


In axial flow, air enters and leaves the fan with no change in direction (propeller, tubeaxial, vaneaxial)Vaneaxial fans are similar to tubeaxials, but with addition of guide vanes that improve efficiency by directing and straightening the flow. Propeller fans usually run at low speeds and moderate temperatures. They experience a large change in airflow with small changes in static pressure. They handle large volumes of air at low pressure or free delivery. Propeller fans are often used indoors as exhaust fans. Outdoor applications include air-cooled condensers and cooling towers. Efficiency is low approximately 50% or less.

The major types of centrifugal fan are: radial, forward curved and backward curved.Radial fans are preferred because of their high static pressures (upto 1400 mmwc).They

are simple in design and can handle heavily contaminated airstreams , high temperatures and medium blade tip speeds.

Forward-curved fans are used in clean environments, lower temperatures, low tip speed and high-airflow work - best suited for large volumes of air against relatively low pressures.

Backward-inclined fans are more efficient than forward-curved fans. Backward-inclined fans reach their peak power consumption and then power demand drops off well within their useable airflow range. Backward-inclined fans are known as "non-overloading" because changes in static pressure do not overload the motor

Thus, the system resistance increases substantially as the volume of air flowing through the system increases; square of air flow. Conversely, resistance decreases as flow decreases. To determine what volume the fan will produce, it is therefore necessary to know the system resistance characteristics.In existing systems, the system resistance can be measured. In systems that have been designed, but not built, the system resistance must be calculated. Typically a system resistance curve is generated with for various flow rates on the x-axis and the associated resistance on the y-axis.TYPICAL FAN PERFORMANCE CURVE (not for our plant) OF A CENTRIFUGAL FAN.

28


Fan characteristics can be represented in form of fan curve(s). The fan curve is a performance curve for the particular fan under a specific set of conditions. The fan curve is a graphical representation of a number of inter-related parameters. Typically a curve will be developed for a given set of conditions usually including: fan volume, system static pressure, fan speed, and brake horsepower required to drive the fan under the stated conditions. Some fan curves will also include an efficiency curve so that a system designer will know where on that curve the fan will be operating under the chosen conditions. In the many curves shown in the Figure, the curve static pressure (SP) vs. flow is especially important.

The intersection of the system curve and the static pressure curve defines the operating point. When system resistance changes, the operating point also changes. Once the operating point is fixed, the power required could be found by following a vertical line that passes through the operating point to an intersection with the power (BHP) curve. A horizontal line drawn through the intersection with the power curve will lead to the required power on the right vertical axis. In the depicted curves, the fan efficiency curve is also presented.

In any fan system, the resistance to air flow (pressure) increases when the flow of air is increased. As mentioned before, it varies as the square of the flow. The pressure required by a system over a range of flows can be determined and a "system performance curve" can be developed (shown as SC & SC in Figure ).1 2

A fan operates along a performance given by the manufacturer for a particular fan speed. (The fan performance chart shows performance curves for a series of fan speeds.) At fan speed N , the fan will operate along the N performance curve as shown in figure.11

SYSTEM CURVE S FOR 2 AIR REQUIREMENTS Q & Q . 1 2

29


The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM. Fan Design and Selection Criteria :-

Precise determination of air-flow and required outlet pressure are most important in proper selection of fan type and size. The air-flow required depends on the process requirements; normally determined from heat transfer rates, or combustion air or flue gas quantity to be handled.System pressure requirement is usually more difficult to compute or predict. Backward-curved fans provide the most stable operation. Also, the power required by most backward curved fans will decrease at flow higher than design values.

This system curve can then be plotted on the fan curve to show the fan's actual operating point at "A" where the two curves (N1 and SC1) intersect. This operating point is at air flow Q1 delivered against pressure P1.

Operating point on this curve will depend on the system resistance; fan's operating point at "A" is flow (Q1) against pressure (P1).Two methods can be used to reduce air flow from Q1 to Q2 :

First method is to restrict the air flow by partially closing a damper in the system. This action causes a new system performance curve (SC2) where the required pressure is greater for any given air flow.

The fan will now operate at "B" to provide the reduced air flow Q2 against higher pressure P2.Second method to reduce air flow is by reducing the speed from N1 to N2, keeping the

damper fully open. The fan would operate at "C" to provide the same Q2 air flow, but at a lower pressure P3.

Thus, reducing the fan speed is a much more efficient method to decrease airflow since less power is required and less energy is consumed. This is why VFD comes into picture and is described separately in this book.Fan Laws

30


Forward curved fans, however, are less efficient than backward curved fans and power rises continuously with flow. Thus, they are generally more expensive to operate despite their lower first cost. Aerofoil designs provide the highest efficiency (upto 10% higher than backward curved blades), but they tend to develop imbalance due to ash entry in the hollow of blades.

Flow-rate capabilities and performance is also dependant on the fan enclosure and duct design. Spiral housing designs with inducers, diffusers are more efficient as compared to square housings. Density of inlet air is another important consideration, since it affects both volume flow-rate and capacity of the fan to develop pressure. Inlet and outlet conditions (whirl and turbulence created by grills, dampers, etc.) can significantly alter fan performance curves from that provided by the manufacturer (which are developed under controlled conditions). Bends and elbows in the inlet or outlet ducting can change the velocity of air, thereby changing fan characteristics (the pressure drop in these elements is attributed to the system resistance). All these factors, termed as System Effect Factors, should, therefore, be carefully evaluated during fan selection since they wouldmodify the fan performance curve.

The choice of safety margin also affects the efficient operation of the fan In the case of boiler; the induced draft (ID) fan can be designed with a safety margin of 20% on volume and 30% on head. The forced draft (FD) fans and primary air (PA) fans do not require any safety margins. However, safety margins of 10 % on volume and 20% on pressure are maintained for FD and PA fans.

The inlet damper positioning is also to be checked regularly so that the "full open" and "full close" conditions are satisfied. Flow Control Strategies

Typically, once a fan system is designed and installed, the fan operates at a constant speed.

There may be occasions when a speed change is desirable. Various ways to achieve change in flow are: pulley change, damper control, inlet guide vane control, variable speed drive and series and parallel operation of fans.Two fans together side by side:-

Two fans in parallel will result in doubling the volume flow, but only at free delivery. As Figure shows, when a system curve is over-laid on the parallel performance curves, the higher the system resistance, the less increase in flow results with parallel fan operation.

Series operation can be defined as using multiple fans in a push-pull arrangement. By staging two fans in series, the static pressure capability at a given airflow can be increased, but again, not to double at every flow point, as the above Figure displays. In series operation, the best results are achieved in systems with high resistance. Combined performance curves are generally unstable, unpredictable and a function of the fan and motor construction and the operating point. In a power plant ID Fans operate in Parallel mode and FD+PA Fans in Series mode.

31


Series and Parallel Operation of Fans.

32

ELECTROSTATIC PRECIPITATOR(ESP)MAKE-ALSTOM, TYPE-OHTA-58/1150

An electrostatic precipitator is a large, industrial emission-control unit. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process.

An electrostatic precipitator is a large, industrial emission-control unit. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process.


ESP Sectional View

Specification of ESP Flue Gas Flow Rate 60.9 m /sec Avg. Gas Velocity 0.5 m/sec Pressure Drop Across ESP 25 mmWC. Operating Temp- 140 deg. C Inlet Dust Concentration 112 gm/ Nm Outlet Dust Concentration - 50mg/ Nm . Flue gas treatment time 33.85 Sec. Collecting surface area 9576 M . Current density- 0.3 Amps/M

3

3

3

2

2

Specification of ESP3

Flue Gas Flow Rate 60.9 m /sec Avg. Gas Velocity 0.5 m/sec Pressure Drop Across ESP 25 mmWC. Operating Temp- 140 deg. C

3 Inlet Dust Concentration 112 gm/ Nm

3 Outlet Dust Concentration - 50mg/ Nm . Flue gas treatment time 33.85 Sec.

2 Collecting surface area 9576 M .

2 Current density- 0.3 Amps/M

3.6 ESP

33

Construction Details of an ESP

WORKING PRINCIPLE


Basic Principles Electrostatic Precipitation is the most effective method for removal of solid dust particles from the industrial flue gases. The Electrostatic Precipitator consists of a large steel chamber and the electrode system. The high voltage DC supply applied between the emitting and collecting electrodes, with the emitting electrode connected to the negative terminal. The collecting electrodes are connected to the positive terminal, which earthed. When flue gas pass through the chamber because of the high DC voltage the solid particles are ionized and travel towards collecting electrodes and collected. The rapping mechanism raps the collecting electrodes and the dust drops in the hoppers. The dust collected in hoppers is finally disposed off.

34


Six activities take place in ESP

Corona Discharge formation.

Ionization of Particles.

Corona formation. Ionization - Charging of particles. Migration - Transporting the charged particles to the collecting surfaces Collection - Precipitation of the charged particles onto the collecting surfaces Charge Dissipation - Neutralizing the charged particles on the collecting surfaces Particle Dislodging - Removing the particles from the collecting surface to the hopper Particle Removal - Conveying the particles from the hopper to a disposal point.

When the potential difference between the wire and plate electrode increases, a voltage is reached when an electrical breakdown of gas occur near the wire. When the gas molecules get excited, electron shift towards the high energy level. The bluish glow around the wire is called Corona Discharge. An advance state of corona discharge is spark break down. Corona discharge take place at (peak) DC voltage of 50 to 95KV.

ESP operates on Negative Corona (Ionization electrode is at Negative Polarity)

The field strength is high near the emitter electrode and gradually reduces. The space between wire and plate can be divided in to an ACTIVE zone & PASSIVE zone.

In Active zone the electric field strength is vary strong, which accelerate the free electrons leaving

from the emitter. These electrons acquires sufficient energy to ionized molecules in gas upon

collision, producing +ve charge particles & additional electrons. These additional electrons

35


through the particles. The flow of current is function of dust particle resistivity. also accelerate and ionize more gas molecules. And the chain reaction continue.until the field strength decreases to the point that the released electrons do not acquire sufficient energy for ionization. The positive ion are attracted towards the ve electrode. Deposition of +ve ions reduces

the emission of electrons, Periodic cleaning of electrode by rapping mechanism will free the particle and collected in the hopper.

Passive Zone:- Electrons from active zone enters into passive zone. Here the field strength are insufficient for further ionization.

The Gas molecules passing through the inter electrode space are subjected to intense bombardment of electron released from active zone. These ions collide with gas particles and charged them.

Flue Gas molecules being electronegative (strong affinity towards electrons or has a tendency to absorb electron). Due to composition of Coal ash mainly consist of SiO2 (40 to 65% by wt) & Al2O3 (25 to 40% by wt) and few other elements. Oxygen in Sio2 & Al2O3 makes it highly electronegative.

Particles charging is depends upon the size of particles. Particles size above 1 m charging by Field charging. Particles size below 0.2 m charging by diffusion charging ( due to the random collision

of particles). These ionized gas molecules move towards the collecting electrode driven by the

electrostatic field and attain a velocity known as migration velocity.

Charging of Particles.

Migration of Charged Particles.

Collection of Charged Particles.

These ionized gas molecules move towards the collecting electrode driven by the electrostatic field and attain a velocity known as migration velocity.

The charged gas particles are only lightly held on the collecting electrode. The force that holding the particles on collecting surface result from the flow of current

36


For better collection efficiency the resistivity of dust particles should in range of 105 to 1010 -cm.

Due to high carbon in flue gas & Large particle size, Resistivity of particle is too low (ie below 105 -cm). Then not enough charge is retained by the particles and the particles re appear in the Gas stream.

If the resistivity is too high (ie above 1010 -cm), then the flow of current is not enough to hold the particle on the collecting plate.

Dislodging of particles from the collecting electrode is done by Rapping Mechanism. Gas particles form a layer (cake) on the collecting electrode. When rapped loose from the

collecting plate, fall as a COARSE Aggregate. So that it can not re-appear into gas stream.

Rapping cycle timing is maintained, to provide sufficient time for the layer formation on the plate.

The Dust load at bottom is higher. Bottom Rapping is more effective.

The rapping acceleration required for Collecting electrode is around 100 to 150g for Collecting Electrode (where g=9.8m/s2 is acceleration due to gravity).

The rapping acceleration for emitter electrode is around 50g.

Dislodging of Particles.

37


BACK CORONA

ACE-16 CONTROLLER SETTING PARAMETERS

Back Corona discharge is formed by a series of micro discharges in the air space between the dust particles deposited on the collecting plate.

Causes of Back Corona Due to improper rapping impact or cycle time. High resistivity ash due to the use of low sulfur coal. If the layer of collected materials is allowed to reach a thickness more than desirable

before rapping the plate (due to failure of rapping system or in correct rapping cycle time). The thick layer of dust on collecting plate act as an insulating layer. When its resistivity

exceed a threshold (more than 1011 -cm) than that in gas space due to ions flow. The high resistivity increase ions holding capacity of dust layer.

The voltage drop across the dust layer becomes become so high cause sparking (ie Vb>threshold voltage) in the dust layer. Threshold voltage is the max voltage that it can withstand with out breakdown.

But here discharge takes place from +ve to ve, which causes emission large amount +ve ions in to the space. Hence, named as Back Corona.

These +ve ions in free space discharge negatively charged ions. As a result neutralizing of charged particles take place.

Thus reduce the collection efficiency.

The ACE-16 is based on Intel 8088, which has an internal 16-bit architecture with external 8-bit data interface.

The controller unit regulates the avg. value of the current in a closed loop control system. The current and thereby the corona discharge voltage is adjusted by means of thyristor

control. Thyrister control the AC voltage applied to high voltage rectifier Xmer unit (HVRT)

38

Arc Detection and Quenching


The output voltage depends on firing angle . varies from 120 to 150.

Secondary of HVRT is rectified by full wave rectifier ckt. After every spark Voltage/Current is brought down to zero.

And maintained it at zero for preset period. Voltage again built up at a fast rate to the predefined value. This method of control result in effective spark quenching

and at the same time helps in maintaining a higher average voltage.

39

IMPORTANT COMPONENTS

High Voltage Transformer Rectifier Unit. MAKE--HIND RECTIFIER LTD. RATING- 108 KVA. Sr.No- 2005K17134/A/ 1 to 4. Input 360 V/ 2Phase / 300 Amps /50Hz. Output - 67KV / 1.61 Amps. DC Peak Voltage 95KV/1150 mA. Rectifier Unit Full wave Bridge Rectifier.

Emitter Electrode (Spiral type). Material- special stainless Steel UHB 904L. Quantity Installed 4 fields x 1026 nos.= 4104

Nos. Diameter of Wire 2.7mm. Effective length (each spiral unit) 5 Mtrs. Distance between Emitter Distance between emitter & Collecting Plate-

200 mm.

Collecting Plate. Material - Cold Rolled Sheet. Shape Modified G Profile. (for better

performance). Thickness 1.25mm. Active Height 14 Mts. Distance between Plate 400 mm. No. of Gas Passage / Stream 19 Nos. No. of Electrode Installed - 4 field x 120 Nos.=480Nos

Rapping Assembly & Gas Distribution Plate No. of Rapper Units- Emitter Electrode- 4 Nos. Collecting Electrode 4Nos. Distributor plate 1No.


40

Controller. Microprocessor Based

Support Insulator. Material Alumina. Shape- Conical. Leakage Distance 370 mm Flash over voltage 125 KV

Ash Collecting Hopper No. of Hopper 4. Capacity of Hopper 87 m3 Valley angle 55.

Dry Ash evacuation system. Make Macawber Beekay Pvt. Ltd. Capacity of Field I 23 CFT (0.65 m3) / Cycle. Capacity of Field II 23 CFT (0.65 m3) / Cycle. Capacity of Field III 12 CFT (0.34 m3) /

Cycle. Capacity of Field IV 6 CFT (0.17 m3)

/Cycle.


41


3.7 Ash handling plant

GENREALAsh generated from the boiler is collected at four location of fluidized bed , economizer

hoppers , air heater hopper and electrostatic precipitator hoppers.The fludised bed is providec with 2 nos bottom dains per compartment one for manual

and one is provided with bed ash cooler . the bottom ash drains shall be used for emergency purpose and when the bed build up rate is high .Excessive pressure drop in the bed shall be used as an indication to operate the bottom drains of bed.

The economizer bottom is provided with 2 nos of each hopper with isolation gate arrangement . The over flow pipe provided in the ash hopper with isolation gate arrangement to facilitate the disposal of ash while ash handling system is in maintaince.

BED ASH SYSTEMThis boiler is having eight compartment and each compartment is having one no. of ash

Drain point. The maximum designed ash collection rate per compartment drain pipe is 50 kg/hr. The ash drain pipe has been connected to the bed ash cooler, Where the bed ash of 900 C will be cooled to 175 C. Each ash drain pipe will be having separate ash discharge point and these ash discharge point will be connected to ash handling system. The ash drain will be having a by pass discharge point for any emergency drain. As these emergency drain are not connected with coolers the ash drained through this drain will be having a temperature of 900 C [ ie the actual bed operating temperature] the bed ash cooler is same as like of FBC system. The atmospheric air has been tapped at the outlet of F. D. fan and fed through the air nozzles provided in the ash cooler, where the air and bed ash will be mixed and get cooled. The cooled ash has to be collected through the ash handling system and it can be conveyed to storage silo.The hot air from the ash coolers have been connected through ducts to air heater flue gas outlet.In case of ash cooler maintenance/failure,the ash can be removed manually through the bottom of ash drain pipe gate arrangement.

Power cylinder operated shutoff dampers are provided at the air inlet of ash cooler. These dampers can be operated from the DCS panel either manually or automatically. To accomplish the above, individual pressure transmitters are provided in each air box compartments so that whenever the air box pressure is getting increased due to the buildup of bed ash materials, an alarm will be generated to indicate the operator to perform the task manually .The same can be operated automatically considering the programs done in the DCS according to the logics provided.

The sequence of cooling of bed ash should be such that ash in alternate compartment is cooled repeatedly. The operation sequence is decided in such a way that only one compartment can be operated at a time.

42


Air inlet in the bed ash coolers is controlled by the power cylinders whose operating time is decided and set in the DCS. The compartment damper (i.e. ash cooler air inlet damper) will be opened only for a pre-set time.

When the time is elapsed the adjacent compartments damper will be opened. The compartment not in operation shall not be considered in the sequence of operation.

The timing sequence (i.e. one cooler to other) can be 5 minutes (maximum) and it should be decided during commissioning of the cooler.

4. Steam TurbineSteam Turbine

43


Introduction : The steam turbine can be said to resemble a steam windmill. Pressurized steam is accelerated through a nozzle and then directed (almost) tangentially onto blades attached to a rotating wheel. Torque is generated by reaction forces as steam is redirected by the blades. There must always be some axial velocity so the redirected flow of steam can make way for the incoming flow. Axial-flow turbines have a rotor, which has number of disc wheels (of gradually increasing diameters) shrunk on to a shaft, and sets of circumferential rows of blades mounted on it. A conical casing encases the rotor and has rows of fixed blades mounted on it. The casing blades remain stationary, and are also known as stator Diaphragms or guide blades. Rotor blades are set in alternate rows with the stator blades. Steam is admitted at one end of the turbine, and travels in the space between rotor and casing through sets of fixed and moving blades to the exhaust where it passes to a condenser. Steam flow is directed by nozzles or fixed blades onto the moving blades, providing a turning moment. As steam passes through the turbine, its pressure decreases and its specific volume increases. The flow area between rotor and casing therefore needs to increase progressively, and the blades become longer. Clearances between moving blade tips and the casing, and between the fixed blade tips and the rotor, are kept small to reduce leakage through the gaps, so as much steam as possible impinges on the blades. As a result, there is less friction and vibration in a turbine.Virtually all modern steam turbines are axial-flow

To maximize turbine efficiency the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known

44


as either impulse or reaction turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as either one or the other, but the overall turbine uses both. Typically, higher pressure sections are impulse type and lower pressure stages are reaction typeImpulse turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.

As the steam flows through the nozzle its pressure falls from inlet pressure to the exit pressure . Due to this higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss".Reaction turbines

In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.Features of Steam TurbinesImpulse and Reaction Turbines

There are three principal types of axial-flow turbines: impulse turbines, reaction turbines, and a combination of the impulse and reaction turbine contained in the same casing.

The main difference between impulse and reaction turbines is in the action of the steam and the manner in which the heat energy is converted into mechanical energy.

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Impulse Turbines :In an impulse turbine, steam passes through stationary converging (subsonic) or converging-diverging (super-sonic) nozzles, which reduce its pressure (and its temperature) and increase its velocity, thus converting its "heat energy" (enthalpy) into kinetic energy. These nozzles direct steam at high velocity onto curved blades which are attached to the rotor. Steam leaves the blades at an angle similar to that of its entry, but in the opposite direction with respect to the axis of wheel rotation. The change in momentum due to deflection of the steam produces a corresponding force on the blades, using the steam's kinetic energy to drive the turbine shaft.

However, if the steam were allowed to expand from boiler pressure to condenser (exhaust) pressure through just one set of nozzles, the excessively high velocity developed could not be used efficiently without running the turbine at excessive speeds. Hence only 1 Impulse stage is provided followed by reaction stages.

Reaction TurbinesReaction turbines use the principle of the reaction force. The reaction principle

states that the force required to accelerate a body has an equal and opposite reaction In a reaction turbine the fixed and moving blades are shaped such that the passages between the blades act as nozzles.

Therefore there is a pressure drop across both the fixed and moving blades.(Note: in impulse turbines there is no pressure drop across the blades

either fixed or moving). In reaction turbines, the fixed blades are identical in shape to the moving blades,

As steam moves through the moving blades, it gives them an impulse. The velocity increases in the converging (fixed) passages between the fixed blades, and the steam imparts an impulse force to the fixed blades. Since the blades are fixed, there is an equal and opposite reaction force imparted to the previous moving blades. Therefore there are TWO forces on the moving blades - an impulse force (from the previous fixed blade) and a reaction force (from the next fixed blade) - both of these forces driving the moving blades.

Although the total force driving the moving blades is a combination of an impulse force and a reaction force, this design of turbine is always referred to as reaction turbines. Therefore we have the pressure falling steadily throughout the fixed and moving blades. The drop in pressure is equal across the fixed and moving blades. Unlike impulse stage there is pressure drop in moving blades in reaction stages and therefore steam tends to leak around the blade tips.

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4.1 Turbine Governing SystemGOVERNING SYSTEM: OVERVIEW

Governing system is an important control system in the power plant as it regulates the turbine speed, power and participates in the grid frequency regulation. For starting, loading governing system is the main operator interface. Steady state and dynamic performance of the power system depends on the power plant response capabilities in which governing system plays a key role. With the development of electro- hydraulic governors, processing capabilities have been enhanced but several adjustable parameters have been provided. A thorough understanding of the governing process is necessary for such adjustment.Need for governing system

The load on a turbine generating unit does not remain constant and can vary as per consumer requirement. The mismatch between load and generation results in the speed (or frequency) variation. When the load varies, the generation also has to vary to match it to keep the speed constant. This job is done by the governing system. Speed which is an indicator of the generation load mismatch is used to increase or decrease the generation. Basic scheme

Governing system controls the steam flow to the turbine in response to the control signals like speed error, power error. It is a closed loop control system in which control action goes on till the power mismatch is reduced to zero. Inlet steam flow is controlled by the control valve or the governor valve. It is a regulating valve. The stop valve shown in the figure ahead of control valve is used for protection. It is either closed or open. In emergencies steam flow is stopped by closing this valve by the protective devices.

The governing process can be functionally expressed in the form of signal flow block diagram shown. The electronic part output is a current signal and is converted into a hydraulic pressure or a piston position signal by the electro- hydraulic converter (EHC). Control valves are finally operated by hydraulic control valve servo motors.

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The steam flow through the control valve is proportional to the valve opening in the operating range. So when valve position changes, turbine steam flow changes and turbine power output also changes proportionally. Thus governing system changes the turbine mechanical power output.

In no load unsynchronized condition, all the power is used to accelerate the rotor only (after meeting rotational losses) and hence the speed changes. The rate of speed change is governed by the inertia of the entire rotor system. In the grid connected condition, only power pumped into the system changes when governing system changes the valve opening. When the turbine generator unit is being started, governing system controls the speed precisely by regulating the steam flow. Once the unit is synchronized to the power system grid, same control system is used to load the machine. As the connected system has very large inertia ('infinite bus'), one machine cannot change the frequency of the grid. But it can participate in the power system frequency regulation as part of a group of generators.

As shown in the block diagram, the valve opening changes either by changing the reference setting or by the change in speed (or frequency). This is called primary regulation. ELECTRO HYDRAULIC GOVERNING SYSTEM:-Basically the controls can be described as i) speed control when the machine is not connected to the grid or in isolation and ii) load control when the machine is connected to the grid.

The governing system has three functional parts: i) sensing part ii) processing part and iii) amplification. These functions are realized using a set of electronic, hydraulic and mechanical elements, in the electro-hydraulic governor (EHG), as shown in Fig.in next page.

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Earlier, only mechanical-hydraulic elements were employed in mechanical-hydraulic governor (MHG). With the developments in electronics technology, the microprocessor- based and digital signal processor (DSP) based governors are being used.

Sensing: to sense speed and power (or MW). The well known fly ball governor is a mechanical speed sensor which converts speed signal in to a mechanical movement signal. Nowadays electronic sensors using Hall Effect principle and/or hydraulic sensor (a special pump whose output pressure varies with pump speed linearly) is used for speed measurement.

Processing: In digital governors the processing is done using software blocks.Amplification is necessary to obtain sufficient power to operate the steam control valve

(where forces due to steam pressure also act)Regulation or droop characteristic

Whenever there is a mismatch in power, speed changes, the governing system senses this speed change and adjusts valve opening which in turn changes power output. This action stops once the power mismatch is made zero. But the speed error remains. What should be the change in power output for a change in speed is decided by the 'regulation'. If 4 % change in speed causes 100 % change in power output, then the regulation is said to be 4 % (or in per unit 0.04).

The regulation can be expressed in the form of power frequency characteristic as shown below. At 100 % load the generation is also 100 %, frequency (or speed) is also 100%. When load reduces frequency increases, as generation remains the same. When load reduces by 50 %, frequency increases by 2 %, in the characteristic shown. When load reduces by 100 %, frequency increases by 4 %. In other words 4 % rise in frequency should reduce power generation by 100 %. This 4 % is called 'droop' of 4 %. The characteristic is of 'drooping' type. Droop or regulation is an important parameter in the frequency regulation. In thermal power plants droop value is generally 4 % or 5 %.

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In terms of control system steady state gain it is expressed as inverse of droop: gain of 25 in per unit corresponds to 4 % (or 0.04 p.u) droop.

In our unit woodward 505E is provided to control the hydraulic servo motor to operate the CV and ESV

Glands are used on turbine to prevent or reduce the leakage of steam or air between rotating and stationary components which have a pressure difference across them; this particularly where the turbine shaft passes through the cylinder.

At the front side of the turbine gland the steam pressure is higher than the atmospheric pressure so there will be steam leakage outward and at the rear side of the turbine glands the steam pressure is below the atmospheric pressure so there will be a leakage of air inwards and it effects condenser performance.

Labyrinth glands are used to prevent shaft leakage with with carbon rings and gland leak-off. Labyrinth seal consists of series of thin strips fixed with the casing which maintains the smallest possible clearance with the shaft, the small construction makes the steam throttled to lower pressure many times, till only a very small quantity leaks.

Our 30MW turbine has got 2 sets of gland seals one at front side and second at rear side.2These glands are sealed by steam from PRDS-2 at a pressure of 1.15 kg/cm abs to 1.10

2kg/cm abs and temperature between 230 to 250C.

4.2 Gland sealing system

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4.3 Air evacuation system

Air leaks into the condenser shell through flanges. Some air also comes along with steam, which has leaked into the exhaust end of the turbine along the shaft.

As the air-water vapour mixture approaches the cold tube surface, water vapour condenses and air being non-condensable gas forms an air film around the condensate.

This air film affects the condenser performance badly because i) it reduces the heat transfer ii) it reduces condenser vacuum and turbine exhaust pressure thus reducing the turbine output.

A Steam Jet Air Ejector (SJAE) is used to remove air from condenser shell. Auxiliary steam from PRDS-2 is used as the motive steam for the ejector. The vacuum created at the nozzle throat draws air from the shell along with some steam and the combined flow gets compressed while flowing through the diffusor part. The escaping vapour is condensed in the inter condenser and the remaining air is again drawn by the vacuum created at the nozzle throat of the second stage. The mixture of motive steam and air is further compressed in diffuser. In the after condenser the vapour is condensed and air, now at a pressure higher than atmosphere, is vented out. Cooling water flowing through both inter condenser and after condenser is the condensate from the hotwell prior to its flow through the LPH.

The Rankine cycle is a thermodynamic cycle which converts heat into work. In a thermal power plant heat is supplied externally to a closed loop, of water as the working fluid. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants.

4.4 Rankine cycle

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Temperature

Entropy

567891011

1213

1415

1

2

3

4

Heat rejected from working fluid

Wor

k tra

nsfe

red

to fe

ed w

ater

Heat transfered to the steam

Work transfered from

steam

1

2 3 4

5

6

7

8

9

10

11

11

12

13 1415

CW I/O

When the all the following four processes are ideal the cycle called Rankine cycle.a) For steam boiler , this would be reversible constant pressure heating process of water to form

steam .b) For turbine the ideal process would be a reversible adiabatic expansion of steam.c) For the condenser it would be reversible constant pressure heat rejection as the steam condenses

till it becomes saturated liquid.d) For the pump the ideal process would be the reversible adiabatic compression of the liquid ending

at the initial pressure.

ECONOMISER,EVAPORATOR AND SUPER HEATER:Heat transfer to water in the steam generator takes place in the three different regimes.1. Water is first heated sensibly in the economiser in the liquid phase at a certain pressure till it

becomes saturated liquid. 2. Evaporator or Boiler : phase change or boiling with the state changing by absorbing the latent

heat of vaporization at that pressure.3. Super heater : The saturated vapour is further heated at a constant pressure in vapour/gaseous

phase.

These states are identified by number in the above diagram. Process 6-7: CEP; 9-10: BFP: Water is pumped from low to high pressure, at the expense of

little input energy (@2.2%). Process 10-11-12: The high pressure water enters a boiler where it is heated at constant

pressure by coal combustion to become a dry saturated vapour and then to superheated steam i.e 13-14-15-1.

Process 1-2-3-4-5: The superheated steam expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.

Process 5-6: The wet vapor then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling water and efficiency of heat transfer. Only latent heat is to be removed.

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In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine would generate no entropy and hence maximize the net work output. Processes 6-7 and 9-10 would be represented by vertical lines on the T-s diagram and more closely resemble that of the Carnot cycle. This is a general way of showing the power plant process and modifications due to Bleed

Steam for Regenerative Feed Water Heating. There would be a further modification in a Reheat Cycle.

FEED WATER HEATERSTo improve the cycle efficiency regenerative feed water heaters are used in steam power

plants, to raise the feed water temperature before it enters to the economiser.Feed water heaters are shell and tube heat exchanger. The heaters receive bled steam from

the turbine at pressure determined by equal temperature rise from the condenser to the boiler saturation temperature.

Our 30MW unit have Two heaters i) Low pressure heater (LPH), ii) High pressure heater(HPH)

LPH is located between the condensate pump and the deaerator and the HPH is located between the BFP and Economiser.

When bled steam entering a feedwater heater is superheated, as in a HPH, the heater includes a desuper heating zone where steam is cooled to its saturation temperature. It s followed by a condensing zone where the steam is condensed to a saturated liquid rejecting the latent heat of condensation. This liquid is called heater drain, is than cooled below its saturation temperature in a sub cooling zone before the drain is cascaded backward.LPH drain to condenser flash box and HPH drain to deaerator.

DEAERATORDeaerator is used to remove the

dissolved gases from feed water used in generating steam and it is also is one of the feedwater heater is a contact type open heater. Why gases need to be removed from boiler feedwater

The presence of oxygen, Carbon dioxide in feed water makes the water corrosive, as they react with the metal to form iron oxide. Particularly at elevated temperature the solubility of these gases in water decreases with increase in temperature and becomes zero at the boiling or saturation temperature. So these gases are needed to be removed from the system. This is achieved by embodying into the freed system a deaerating unit.

4.5 Regenerative heating system

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Typically a feedwater inlet to deaerator should be operated at 85C to 90C. This leaves an oxygen content of around 2 mg / litre (ppm).The addition of an oxygen scavenging chemical (sodium sulphite, hydrazine) will remove the remaining oxygen and prevent corrosion.

This system reduces dissolved oxygen concentration to less than 0.005 cc/liter (7 ppb), and completely eliminates the carbon dioxide concentration.

Our 30 MW unit Deaerator, pegged at 4.0 kg/cm(abs) is provided in turbine regenerative cycle to provide properly deaerated feedwater for boiler, limiting gases (mainly oxygen) to 0.005cc/Litre. It is direct contact type heater with feed storage tank of 20 MT capacity.

The heating steam is normally supplied from turbine extraction but during starting and low load operation the steam is supplied from main steam line through PRDS-1.

Deaerator comprises of two chambers: Deaerator column and feed storage tank.Deaerating column: It is a spray cum tray type cylindrical vessel of vertical construction

mounted on feed storage tank. The tray stack is designed to ensure maximum contact time to achieve efficient deaeration. The feedwater is admitted at the top of the deaerating column and flows downwards through the spray valves and trays. Steam enters from the underneath of the tray and flows in counter direction of the condensate. while flowing upwards through the trays, scrubbing and heating is done, thus the liberated gas move upward along with the steam. Steam gets condensed above the trays and liberated gases escape to atmosphere from the orifice.

Location of Deaerator:

A deaerator is placed at a height of about 13.5 mts above BFP suction to avoid flashing and cavitation during a rapid load drop. During a load drop turbine bleed steam pressure in the heaters and deaerator tends to drop.This causes flashing in the deaerator as the water is stored at boiling point,corresponding to the pressure a

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30 MW POWER PLANT