VOCATIONAL TRAINING

VOCATIONAL TRAINING

On500MW THERMAL POWER PLANTAT

NTPC LIMITED

RIHAND SUPER THERMAL POWER PROJECT,

RIHAND NAGAR SONEBHADRA (U.P.)

BY

DINESH SINGH RINGWALOF

PUNJAB POLYTECHNIC COLLEGE,

LALRU, DISTT. MOHALI (PB.)UNDER THE EXPERT GUIDANCE OF:

Mr. P.Kashyap, D.G.M.(BMD)

Mr. Vikas kumar, Sr.Engr.

CERTIFICATE

NATIONAL THERMAL POWER CORPORATION LTD.

(A GOVERNMENT OF INDIA ENTERPRISE)

RIHAND SUPER THERMAL POWER PROJECT

P.O.-Rihand Nagar, Distt. Sonebhadra (U.P.)

.

DATE-23/07/2006

CERTIFICATE

TO WHOMSOEVER IT MAY CONCERN

CERTIFICATE

This is to certify that this summer training report has been prepared by DINESH SINGH RINGWAL S/O Sh. Trilok Singh, a student of DIPLOMA. (MECH. ENGG) in the partial fulfillment of requirement of vocational training at NTPC Rihand.

The matter was analyzed by candidate under my guidance and this report embodies the work done by him.We wish him a bright future.

Approving Authority:

Guide:

A.K.TIWARI

P.K. CHOUDHARYDGM (BMD)

ENGINEER (BMD)

ABOUT N.T.P.C.

The NTPC was formed in 1975 to meet the power requirement challenge faced by the nation during the period. N.T.P.C. (National Thermal Power Corporation) is an Indian Govt. undertaking. It has the installed capacity of 23,745MW including joint venture, contributing 26.7% of the nations power generation with only 19.1% of Indias total installed capacity.

Based on 1998 data, carried out by Data monitor UK, NTPC is the 6th largest in terms of thermal power generation and the second most efficient in terms of capacity utilization amongst the thermal utilities in the world.

INCLUDEPICTURE "http://ntpc.co.in/images/indian_map.jpg" \* MERGEFORMATINET

NTPC's core business is engineering, construction and operation of power generating plants and also providing consultancy to power utilities in India and abroad. As on date the installed capacity of NTPC is 23,749 MW through its 13 coal based (19,480 MW), 7 gas based (3,955 MW) and 3 Joint Venture Projects (314 MW). NTPC acquired 50% equity of the SAIL Power Supply Corporation Ltd. (SPSCL). This JV company operates the captive power plants of Durgapur (120 MW), Rourkela (120 MW) and Bhilai (74 MW). NTPC is also managing Badarpur thermal power station (705 MW) of Government of India.Recognising its excellent performance and vast potential, Government of the India has identified NTPC as one of the jewels of Public Sector Navratnas- a potential global giant. Inspired by its glorious past and vibrant present, NTPC is well on its way to realise its vision of being one of the worlds largest and best power utilities, powering Indias growth.Rihand Super Thermal Power Project

(4x500 MW)

INTRODUCTION

2.1 SALIENT FEATURE

Location: Bijpur village, Distt. Sonebhadra. (U.P).

Total proposed capacity:3000 MW, in 3 stages each of 2X500MW

Present capacity: 2000 MW (STAGE-1 & STAGE-2)

(STAGE-3 is planned for future)

Total land (in acres): UP MP Total

4680 1752 6432

Power Evacuation: +/-500Kv HVDC Bipolar line to Dadri (Delhi), 400kV single circuit AC line to Shaktinagar and Kanpur.

Beneficiary States: UP, Haryana, Punjab, Rajasthan, Jammu&Kashmir,

Himachal Pradesh, Chandigarh and Delhi.

Financing: Export Credit of pound sterling 344million, aid from Govt. of U.K.pound sterling 177 million.Govt.of India afforded the rest cost of the project.

Major Resources:

COAL- (a) Source-Amlori mines.

(b) Maximum consumption- 43,300MT/Day

for 3000 MW (E-Grade Coal).

(c) Mode of Transportation- MGR Rail

Transportation System.

WATER- (a) Source- Rihand Reservoir.

(b) Maximum Consumption 300 cusecs.

(c)Maximum cooling water requirement-

1500cusecs for 1000MW.

Chimney: 224.5mts(RCC structure with steel flue).

Ash disposal: Ash slurry pumped to Ash dyke.

Commencement of work: 09/02/1983.

Unit-1 synchronized: 31/03/1988.

Unit-2 synchronized: 05/07/1989.

Unit-3 synchronized: 31/01/2005

Unit-4 synchronized: 31/07/2005

Unit-1 commercialized 01/01/1990

Unit-2 commercialized: 01/10/1991.

Rihand super thermal power project (RhSTPP) has the current generation capacity of 2000MW. There are four Units where each Unit has the capacity of 500MW. Units 1 & 2 were commissioned in 1989 and Unit 3 & Unit 4 were commissioned recently in August 2005. The power plant has two switchyards of 400KV & 132KV which are interconnected to each other. These switchyards feed the northern power grid form where the transmission of power is done. One 400KV bus is used for H.V.D.C. transmission which delivers power from Rihand to Dadri near Delhi. The thermal plant intakes water from the near by water reservoir the Govind Vallabh Pant Reservoir based on Rihand river. The reservoir rests on the dam named Rihand dam, which also powers the Hydro power station of 250MW. The requirement of coal is fulfilled from Amlori coal mines which are situated in Madhya Pradesh.

The Rh.S.T.P.P. powers various States such as Uttar Pradesh, Delhi, Punjab, Himachal Pradesh, Rajasthan etc.

New technology

Super critical technology at NTPC Sipat project (3X600MW) to increase the efficiency of the cycle and to decrease the green house gas emission.

Closed cycle seawater cooling at Simhadri project for first time in India.

Introduction of IGCC (Integrated gasified combined cycle) for clean and efficient utilization of coal.

Environment management

Liquid water treatment plants at Farakka and Kahalgaon.

Ash water recycling system at Kahalgaon and Korba to reduce water requirement for ash disposal at these station

MODIFIED RANKINE CYCLE

A thermal power station operates using a closed steam power cycle, which is a dual (vapour+liquid) phase cycle. In this cycle, the working fluid i.e. water undergoes various thermodynamic processes and is used repeatedly. Previously, rankine cycle was used in most of the thermal power plants. However, in modern power plants the rankine cycle is modified to include superheating of steam, regenerative feed water heating and reheating of steam.

3 5

2 4

T

1 6

S

The modified cycle is represented on T-S diagram. Various processes are:

1-2 Ideal pressure increase at constant entropy in boiler feed pump.

2-3 Heat addition in the boiler at constant pressure.

3-4 Isentropic expansion in HP turbine.

4-5 Reheating at constant pressure.

5-6 Isentropic expansion in IP and LP turbines.

6-1 Extraction of latent heat in condenser.

Factors affecting thermal efficiency of Modified Rankine Cycle are listed as under:

Initial Steam Pressure.

Initial Steam Temperature.

Reheat Pressure and Temperature.

Condenser Pressure.

Regenerative Feed Water Heating.MAIN BOILER

A . BOILER FUNDAMENTALS

Principles of combustion:

1. The primary function of oil and coal burning systems in the process of steam generation is to provide controlled efficient conversation of chemical energy of fuel into heat energy, which is then transferred to the heat absorbing surfaces of the steam generator. When combustion is properly completed the exhaust gases will contain, CO2, water vapour, sulphur dioxide and a large volume of nitrogen, combining carbon and hydrogen with the oxygen in air brings about combustion. When carbon burns incompletely it forms carbon monoxide.

2. Composition of air:

a) 79% nitrogen and 21% oxygen by volume.

b) 77% nitrogen and 23% oxygen by weight.

3. Excess air: In practice the theoretical quantity is not sufficient to ensure complete combustion and extra air has to be supplied. This excess air is known as excess air.

4. Ignition:

Fuel must be ignited before it can burn. Raising the temperature of the fuel to its ignition temperature brings about combustion.

5. The following factors in efficient combustion are usually referred to as The three TS.

a) TIME: It will take a definite time to heat the fuel to its ignition temperature and having ignited, it will also take time to burn.

b) TEMPERATURE: A fuel will not burn until it has reached its ignition temperature. Preheating the combustion air increases the speed at which this temperature will be reached.

C) TURBULENCE: Turbulence is introduced to achieve a rapid relative motion between the air and the fuel particles. This produces a quick propagation of the flame and its rapid spread throughout the fuel/air mixture in the combustion chamber.

6) Combustion efficiency:

Maximum combustion efficiency depends on

c) Design of the boiler

d) Fuel used

e) Skill in obtaining combustion with the minimum amount of excess air.

7) Thermal efficiency of a boiler is measured by the amount of the heat transferred to the water in the boiler by each Kg of the fuel used.

ARRANGEMENT OF THE MAIN BOILER:

1). Boiler Structurals:

The boiler structurals are divided into two parts.

a) Supporting Structures: boiler-supporting structure consists of a systematic arrangement of columns stiffened with horizontal beams and vertical diagonal bracings and comprise of low carbon steel material. The main columns support the main boiler components viz. drum, economizers air preheaters, burners and galleries at various levels.

b) Galleries and Stairways: Galleries and stairways around the combustion and the heat recovery areas are provided for approach to the boiler.

2) Furnace:

A Boiler furnace is that space under or adjacent to a boiler in which a fuel is burned and from which the combustion products pass into the boiler proper. It provides a chamber in which the combustion reaction can be isolated and confined so that the reaction remains under controlled force. In addition, it provides a support or enclosure for the firing equipment. The wall construction of the furnace is water-cooled walls i.e. the wall is of tube in which water is flowing continuously

3) Boiler drum:

The boiler drum forms a part of the circulation system of the boiler. The drum serves two functions, the first and primary one being that of separating steam from the mixture of water and steam discharged into it. Secondary,the drum houses all equipments used for purification of steam after being separated from water. The purification equipment is commonly reffered to as the drum intervals.

The quantity of water contained in the boiler below the water level is relatively small compared to the total steam output. Primarily, the drum size is determine by the space required to accommodate the steam separating and purifying equipments. The steam space provided should be sufficient to prevent priming and foaming.

Material: The boiler drum is made of carbon steam plates. The materials used should comply with the Indian Boiler Regulations. Comparing carbon steel and alloy steel as material for drum, the carbon steel costs less per ton of material but the overall weight of drum will be higher because that of thickness.

DRUM INTERVALS:

Drum intervals are used to separate water from steam and to direct the flow of water and steam in a manner so as to obtain the optimum distribution of drum matal temperature in boiler operation.

The arrangement of drum normally consists of two or more integrated device, each of which may be quite different in design and operate on totally different principles. Each stage must have a higher separation efficiency. The greater the number of stages of separation,the lower the required efficiency of each stage .Thus, two stages at 99% efficiency,three stages at 90% efficiency and six stages at 70% efficiency will give similar results.

Water level gauge is mounted on each end of the steam drum. If water level goes outside of the prescribed operating limit then the boiler is tripped.Thus,with increase in pressure,the separation of water from steam by simple devices become more difficult.It become necessary to use more efficient apparatus if primary separation is to be achieved in a confined area.

4) Economiser:

The purpose of the economizer is to preheat the boiler feed water before it is introduced into steam drum by recovering heat from the flue gases leaving the boiler. Entering first, the economizer water is heated to about 30 to 40 deg C below saturation temperature. From economiser the water enters the drum and thus joins the circulation system. Water entering the drum flows down through the down comer and and enters ring header at the bottom. It is located in the boiler second pass below the superheater. The economizer is continuous loop type and water flows in upward direction and gas in the downward direction. All tube circuits originates from the inlet header and terminate at outlet header which are connected with the economizer outlet headers through three rows of hanger tubes.

5)Downcomers:

Down comers provide a passage for water from the boiler drum to bottom ring header. From bottom ring header the water goes to water walls for heat absorption and conversion into steam heating .To achieve the circulation of water into water wall Boiler circulation pumps are provided in down comers.

6)Waterwalls:Water walls are the necessary elements of the boiler. They serve as the means of heating and evaporating the feed water supplied to the boiler from the economizers via boiler drum and down comers. In large boilers, water walls completely cover the interior surfaces of the furnace providing practically complete elimination of exposed refractory surface. They usually consist of vertical tubes membrane and are connected at the top and at the bottom to headers. These tubes receive water from the boiler drum by means of down comers connected between drum and water walls lower header. Water walls absorb 50 percent of the heat released by the combustion of fuel in the furnace, which is utilized for evaporation of feed water. The mixture of water and steam is discharged from the top of the water walls into the upper wall header and then passes through riser tubes to the steam drum. The design and construction of the water walls depends upon the combustion and steam conditions and the size of the boiler. Orifices installed in the inlet of each water circuit maintain an appropriate flow of water through the circuit.

7)Riser tubes:A riser is a tube through which the mixture of water and steam pass from an upper water wall header to the steam drum.8) Super Heater:

There are three stages of super heater; the first stage consists of horizontal superheater of convection mixed flow type with upper and lower banks located above economizer assembly in the second pass. The upper bank terminates into hanger tubes, which are connected into the outlet header of the first stage super heater. The second stage superheater consists of a pendant platen, which is a radiant parallel flow type. The third stage superheater pendant spaced is of convection parallel flow type.

In radiant super heaters heat is absorbed by direct radiation from the furnace and are generally located at the top of the furnace. Since the furnace temperature, and therefore the amount of available heat from radiation, does not rise as rapidly as the rate of the steam flow, thus the steam temperature drops as the steam flow rises.

Convection superheaters absorb mainly by the impingement of flow of hot gases around the tubes. A purely convection superheater has a rising steam temperature characteristic i.e. the amount of available heat from convection, rise as rapidly as the rate of steam flow.

9)Reheater:

The function of the reheater is to reheat the steam coming out from the high pressure turbine to a temperature of 540C. The reheater is composed of two sections-one is front pedant section and rear pedant section. The rear pedant section is located above the furnace arc and the rear water wall and the front pedant section is located between the rear water hanger tubes and the superheater platen section.

The cold reheat is the line from turbine to the boiler and is at a lower than the reheat line from boiler to the turbine called hot reheat steam. Due to resistance of flow through the reheat section, the hot reheat steam is at a lower pressure compared to the cold reheat steam.

10)Burners:

There are forty total pulverized coal burners arranged on the corners at a height of 18 to 25 meters and twelve oil burners provided each in between corner two pulverized two-fuel burner.

The pulverized coal burners are arranged in such a way that ten mills supply the coal to burners at 4 corners, of the furnace. All the nozzles of the burners are interlinked and can be tilted as a single unit from +30 to 30.

The oil burners are fed with the heavy fuel oil till boiler load reaches to about 25%.

11)Desuperheaters:

A. Superheater Desuperheater:

The superheater desuperheater is fitted between stages 4 and 5(on transfer pipe) to control the superheated steam at the specified terminal temperature of 540 oC. The maximum design temperature reduction at the superheater desuperheater is from 446 oC to 388 oC.

The desuperheater comprises a spray nozzle shell and associated spray assembly projecting into a section of the superheater steam line. This section of the steam line forms the desuperheater shell. Steam assisted spray nozzle assembly provides a fine spray of water which attemperates the steam passing through the desuperheater. Spray water for desuperheater is taken from the boiler feed water pump discharge. In addition, spray water regulating stations are provided further downstream in each line.

B. Reheater Desuperheater:

The reheater desuperheater is only brought into use when the reheater outlet temperature rises above the normal temperature.

The reheater desuperheater comprises of a spray nozzle shell and associated spray nozzle assembly projecting into a section of the steam line between the HP turbine outlet and the reheater inlet headers. This section of the steam line forms the desuperheater shell. Water is fed into the shell from the discharge side of the boiler feed pumps via a reheater desuperheater spray water regulating station.

When the reheater desuperheater is called into service water is fed via the water tube and passes through the spray nozzle thereby forming a spray which attemprates the steam passing through the desuperheater and thus decreasing e The quantity of water in the boiler drum is relatively small compared to the total steam output. So, the drum size is determined by the space required to accommodate the steam separating and purifying equipments.

12) Boiler Water Circulations Pumps:

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

13) Igniters:

High Energy Arc type electrical igniters are provided which can directly ignite the heavy fuel oil. An exciter unit stores up the electrical energy and releases the energy at a high voltage and short duration. A spark rod tip, which is designed such a way that converts the electrical energy into an intensive spark. A pneumatically operated retracts mechanism, which is used to position the spark rod in the firing position and retract to the non firing position. Each discrete spark provides a large burst of ignition energy as the current reaches a peak value of the order of 2000 amps. These sparks are effective in lighting of a well-atomized oil spray and also capable of blasting off any coke particle or oil muck on the surface of the spark rod.

B. WATER CIRCULATION SYSTEM:Water must flow through the heat absorption surface of the boiler in order that it is evaporated into the steam. In drum type units the water is circulated from the drum through the generating circuits and back to the drum where steam is separated and directed to the superheater. The water leaves the drum through the downcomers at a temperature slightly below the saturation. The flow through the furnace wall is at saturation. Heat absorbed is the latent heat of vaporization creating the mixture of steam and water.

At very a high pressure the density difference between steam and water becomes progressively less and a point is reached where natural circulation is too slow.Natural circulation is limited to boiler with drum operating pressure around 175 kg/cm-2.

To overcome this forced circulation is employed which uses pumps to speed up the circulation of the water.

TECHNICAL DATA

Main boiler 500MW

Type Balanced Draft ,Dry bottom,Single drum,Radiant Furnace,

Controlled Circulation,Tangentially Fired,Tiltable Nozzle.

Manufacturer M/S BHEL(CE DESIGN) Furnace type Controlled Circulation.

Druma. Overall length 22070 mm

b. Designed pressure 204.9 Ksc.

c. Designed temperature 366 oC.

Superheater

a. Number of stages 6 stages

Reheater

a. Number of stages 3 stages

Steam flow at MCRa. Steam at final superheater outlet 1725 TPH

b. Steam at reheater outlet 1530 TPH

Water flow at MCR

a. Feed water entering economizer 1725TPH

Coal flow at MCR 316.4 TPH

Steam pressure at MCR (ksc)

At final superheater outlet 178

At reheater outlet 43.46

At reheater inlet 45.85

From drum 193.5

Economizer inlet 196.6

BOILER AUXILIARIES

MILLING SYSTEM: -

COAL BUNKER: -

These are in-process storage silos used for storing crushed coal coming from the coal handling plant through conveyor belts. There are ten coal bunkers supplying coal to each mills and are located at top of the mills to aid in gravity feeding of the coal. Each bunker can store coal, which can be used for 12hrs.

COAL FEEDER: -The purpose of coal feeder is to transfer coal at a pre- determined rate, from coalbunker to the mill.

The coal feeder comprises two continuous chains with L sections flight bars mounted between the chains at every fifth link .The chains runs on sprockets mounted at each end of the feeder to given an upper strand movement towards the driven ends and a lower strand movement in the opposite direction. The drive shaft is supported on two self aligning bearing mounted in the Plummer block on support out side the feeder casing, shaft sealing is achieved by the lip seals in the sealing housing and mounted in board of the bearing to abut the feeder casing.

The tail sprocket shaft is mounted in adjustable bearing blocks adjacent to the feeder casing with positioned which allow the feeder chain to be tensioned.

Both upper and lower strands run over full width carrying plates with the lower strands located by angle section guides mounted on the feeder wall. The upper and lower carrying plates and the inside wall are protected from wear by replace by replaceable stainless steel panels, chains are kept clean by rubber wiper. Feeder input is achieved by roller chain drive to the conveyor via a fixed speed electric motor driving a variable speed gear box, torque limiter and fixed out put gear box The electric motor is flanged mounted to variable speed gear box, coupled to the fixed output gear box by a flexible coupling and torque limiter. The principle of operation of coal feeder is that coal flows from the bunker into the chain feeder via feed hopper and is conveyed to the mill feed inlet chute, when the feeder is in the operation, the conveyor chain drag a fixed head of coal towards the driven ends of the feeder. At the end of the carrying plates the coal falls through the conveyor onto the bottom plate, where it is picked up by the returning flight bars and dragged back along the feeder to fall into the mill feed chute.

MILL CHUTE: -

Mill chute trans port coal directly from the coal feeder to its associated mill, in addition an emergency chute is incorporated to allow coal to be removed from the coal bunker via coal feeder in the event that outage time is more than one month, or if there is a danger from fire in the mill chute area, coal feeder, or coal bunker.

Each mill is provided which transport raw coal from the bunker to the inlet of mill at the desired rate. The rate of feeding is controlled by variable speed gear box . Here chain type feeder is present in which continuous chain is moving round sprockets in which a sprocket is driven by a variable speed D.C motor and the other sprocket is a return .Sprocket on this chain ,at different MS plates are connected which are called as scrapers .This type chain feeder are called scraper feeder. The coal from the bunker falls on a platform which is below the scrapper feeder. When the scraper moves it will scrap the coal and at the end of platform the coal falls into the pulverizer.

PULVERISER MILL:-

There are ten mills for every 500 MW unit located adjacent to the furnace at 0 m level. These mills pulverize coal to desired fineness to be fed to the furnace for combustion .

The main structure of the pulverisering mill is fabricated from mild steel in three cylindrical sections, the bottom section (the mill housing support )which support the entire unit and encloses the mill drive gear unit, a center section (the mill housing)that contains the rotary grinding element and upper section (the classifier housing )comprising an accommodate the gas loading cylinders of the mill loading gear .A platform around the upper section provide an access to an inspection door and to the top of the mill routine maintenance and is served by detachable ladder .

.

PULVERIZERS:

The pulverizser exclusive of its feeder, consists essentially of a grinding chamber with a classifier mounted above it. The pulverizing take place in a rotating bowl in which centrifugal force is utilized to move the coal, delivery by the feeder, outwards against the grinding ring (buil ring). Rolls revolving on journals that are attached to the mill housing pulverize the coal sufficiently to enable the air stream through the pulverizer to pick it up. Heavy spring action through the journal saddles, provide the necessary pressure between the grinding surfaces and the coal. The rolls do not touch the grinding rings , even when pulverizer is empty. Tramp iron and other foreign material is discharged through a suitable spout. The air and coal mixture passes upward the classifier with its deflector blades where the direction of the flow is changed abruptly, causing the coarse particles to be returned to the bowl for further grinding. The fine particles, remaining in suspension, leave the classifier and pass on through the coal piping to the windbox nozzles. LOW SPEED MILL:

These are commonly known as tube ball mills and operate at approximately 17 to 20 rev/sec.Such slow speed is essential with these types of mills as otherwise the balls will held along the rorating surface due due to centrifugal force and no milling takes place.

Advantages of low speed mills:

(1). Wearable part which needs replacement between annual overhaulsis only the ball and this can be done when mill is in operation.

(2). No maintenance for long period.

(3). this mill has no relects and does not give tramp iron problem.

(4). There is reverse of fuel within the mill which makes the mill output more stable.

Disadvantages of low speed mills:

(1). power consumed per ton of coal pulverised is nearly to that of economic mills.

(2). This mill consumes more power/ton coal ground especially when not loaded fully as bulk of power is consumed in rotating mass of heavy ball charge.

BOWL MILL:

The ball mill is one of the most advanced design of coal pulveriser.The advantages of this mills are :

(1). lower power consupton.

(2). Reliability.

(3). Minimum maintenance.

(4). Wide capacity(1.7 t/hr. to100 t/hr.).

MILL TECHNICAL DATA:-

Manufacture--------------------------M/S BHEL

Type of pulveriser

1003x RP

No. of Mill / boiler

10

Base capacity of Mill

68Tonnes/Hr.

Speed of pulveriser

42rpm

Normal capacitywith design coal52.5 T/Hr

FANS

A fan is a device by which the air is made to flow at required velocity and pressure in a defined path imparting K.E of its impellers to air/flue gases . This pressure boost is used to create a draught in the air and flue gas system. Fans mainly performs two functions:

i. They supply air required for combustion in the furnace with required pressure & flow.

ii. They evacuate the product of combustion i.e. flue gases into the atmosphere via chimney.

P.A Fan :-

The primary air fan supplies heated air to the coal mills to give dry and pulverized coal to the furnace for efficient combustion.The P.A fan also supply fresh air to prevent coal mills by overheating.

There are two P.A fans per boiler, each fan having pneumatically operated radial guide vanes at the fan inlet to control the fan output. The fan impeller is a double inlet, centrifugal wheel with backward curved plate blades.

Ambient air is drawn into the P.A duct by two 50% duty, motor driven centrifugal fans. The air from each fan discharges into a hot air crossover duct via a steam air heater. This duct extends around to each side of the boiler to supply the hot air to mills duct, both of which are branched to supply hot air to four coal mills.

F.D Fan:

The forced draught fan system is provided to supply air required for pulverized coal combustion in the furnace, air for fuel oil combustion and over fire air to minimize Nox production. The F.D fan system comprises of two single stage axial flow, constant speed, and auto variable pitch fans per boiler. These fans provide pressurized atmospheric air to the boiler for combustion. Ambient air is drawn into the secondary air system by two 50% duty, motor driven, axial flow forced draught fans with variable pitch control. The air from each fan discharges into a hot air crossover duct via a main air heater. This duct extends around to each side of the boiler furnace to form two secondary air to burners ducts. At the sides of the furnace, each duct split to supply air to two corners, then split again to supply air to each of the nineteen burner/air nozzle elevations in the burner box.

I.D Fan system:

The induced draught system comprises of three centrifugal double inlet fans per boiler, two operating and one standby. Each fan unit consists of a backward curved plate bladed impeller, which is driven by an electric motor through a variable speed hydraulic coupling. The I.D fan serves the purpose of evacuating the products of combustion or the flue gases in the atmosphere via chimney. The flue gases after being cleaned in the precipitators is directed towards the atmosphere through the chimney.

Scanner air fan :

Scanner fans are installed in the boiler for supplying continuously cooling air to the flame scanner provided for the flame supervision. Normally one fan remains in service while the other one remains available as standby. Scanner air fan is centrifugal type. It takes suction from F.D fan outlet and boosts the pressure further to provide cooling air to the scanners.

Seal air fan:Seal air fan supplies sealing air, at a pressure higher than that inside the mill, to a brush sealing around the stem of the coal mill yoke casting to prevent coal dust escaping to atmosphere. There are eight seal air fans one per mill to provide sealing air. Each fan is a close coupled, electrically operated, centrifugal fan unit operated as part of the mill operating sequence, which incorporates an auto-start facility.

Purge or cooling air fan

The function of the purge/cooling air system is to provide a continuous supply of clean air to cool all soot blowers in the retracted position, and to purge each retractable soot blower lance whenever it is in the retracted position. There are two fans (one operational, and one standby) to provide purging/cooling air to each of the soot blowers, and also to prevent furnace gas blow-back into the sealing areas. Each fan comprises a close coupled, electrically driven centrifugal fan unit, which initially, is manually operated but which contains an automatic standby.

5.Electrostatic precipitatorsThe ash content in the Indian coal is of the order of 30% to 40%. When coal is fired in the boiler, ashes are liberated and about 80% of ash is carried along with the flue gases. If this ash is allowed to atmosphere, it is hazardous to health. So, it became necessary to incorporate an electrostatic precipitator in the path of the flue gases going in the atmosphere. The electrostatic precipitators are preferred to mechanical precipitators because they are capable of precipitating particles from sub micron to large sizes of particles. The efficiency of the modern ESPs is of the order of 99.9%.

In NTPC Rihand, the gas cleaning plant consists of two ESPs that operate on the exhaust gases from each of the 500 MW steam generators. The electrostatic precipitator consists of a large chamber, which comprises of parallel rows of sheet type collecting electrodes suspended from the precipitator casing with wire type discharge electrodes arranged mid-way between them. At the inlet of the chamber, gas distributor screens for uniform distribution of the gases in the chamber, are provided. The collectors are connected to earth at positive polarity while the discharge electrodes are connected to a high voltage dc supply at negative polarity. When dust-laden gas flows between the electrodes, the corona discharge causes the dust particles to become charged, the particles then being attracted towards and, eventually, deposited on the collector electrodes. This dust falls as the collecting electrodes are continuously rapped through a rapping system and is collected into the pyramid type hoppers, located beneath each collecting electrodes, from where it is removed by the ash handling system.6.Air heaters

Air heater is a heat transferring device in which air temperature is raised by transferring heat from flue gases. Air heaters are capable of reclaiming heat from the flue gases at low temperature levels and thus reducing the amount of heat rejected to chimney. This results in increasing the boiler efficiency. For every 20 0C drop in flue gas exit temperature, the boiler efficiency increases by about 1%. In NTPC Rihand, recuperative type of air heaters is mainly used. In recuperative air heaters, the heating medium i.e. flue gases flows through a closely packed matrix structure and then air is passed through the matrix to pickup the heat. There are two recuperative type of main air heaters for heating up the air from fans, two recuperative type air heaters for mill air heating.

Steam air heaters

This type of air heaters does not utilize the heat from the boiler flue gases and so does not affect the boiler efficiency. These are generally used during starting when the flue gases entering the regular air heater is low and hence further heat extraction is not possible and low temperature corrosion prevails. There are two main steam air heaters and two mill air heaters. Steam air heaters comprises of round tubes carrying steam to be condensed. These heaters are provided with air extraction system so that air entering with steam will not lock and prevent the operation of air heater. In addition, the outlet temperature of air can be controlled easily.

7. Firing systemNTPC Rihand has direct firing system. In this system, a controlled quantity of crushed coal is fed to each bowl mill (pulveriser) by its respective feeders and primary air is supplied from the primary air fans which dries the coal as it is being pulverized and transports the pulverized coal through the coal piping system to the coal burners. In this system, burners are set at each corners of the furnace and are directed to strike the outside of an IMAGINARY CIRCLE at the centre of the furnace. Because the streams of fuel strike each other, extremily good mixing is obtained. Since the body of the flame produced is given a rotary motion is leads to a longer flame travel and gases spread out and fill the combustion.

There are ten pulverizes out of which eight are used and two remains in standby. The raw coal feeders supply 74 TPH of coal to each mill.

The pulverized coal and air discharged from the coal burners is directed towards the center of the furnace to form firing circle. There are 32 tilting, tangentially fired coal burners fitted at the four corners of the boiler at eight elevations.

The secondary air heating system supplies secondary air for combustion in the furnace around the pulverized coal burners and through auxiliary air compartments directly adjacent to the coal burner compartments. There are 16 air-atomizing ignitrons per boiler, which initially ignite the coal and air mixture. Above a predictable minimum loading condition, the ignition becomes self-sustaining. Combustion is completed as the gases spiral up in the furnace.

SOOT BLOWING SYSTEM

Introduction

In RhSTPP, soot deposited in various regions of boiler is removed with the help of soot blowers, which utilize steam as the main working medium. This part is mainly concerned with the existing soot blowing system and the problems arising in the system during operation.

The removal of soot is very necessary for the efficient working of the plant. This is because soot acts as an insulator for the heat transfer taking place between the flue gases and water/steam inside the tubes. On the other side, it increases the metal temperature of superheater and reheater tubes located inside the boiler. This temperature increase may cause damage of the tubes resulting leakages across the tubes. Which, in turn affect the working of the boiler.

So, regular soot blowing is very necessary in the boiler. Similarly, soot is also deposited on the air preheater baskets, which causes plugging of the baskets. Also in case of oil firing during initial light-up unburnt oil may deposit in baskets. This deposited oil may cause fire hazard in air heater. To avoid these problems soot blowing is done in air preheater also.

Soot blowing process:The complete soot blowing process can be divided into following three major sequential parts:

A. Shut down process- This part of the process step checks the close status of all the valves.

B. Warm up process- In this step the valves according to the selected blower group open to provide hot steam to the selected blower group at certain pressure level.

C. Blowing process- In this sequence actual blowing of steam takes place. The blower pairs under currently selected group when comes in to operation carries the steam inside the boiler and continues to blow till it returns to its park position.

Steam blowing scheme:

When blowers go inside the boiler they carries steam, which is made available to the blowers through the section valves dedicated for the blower group. These section valves are all motorized and their positions are sensed by their end limit switches.

Valves are classified in four categories according to their operational involvement. They are listed as follows:

a) Main steam supply valves.

b) Steam pressure control valves.

c) Section valves.

d) Drain valves.

The main steam supply for soot blowing is obtained from two alternative sources each isolated by a separate motorized isolating valve. Only one source can be selected at a time. Now, this steam supply is distributed to four sections of the boiler and each section is isolated by a motorized section valve. Down stream of which a pressure switch monitors the steam pressure.

The air heaters can be cleaned using an auxiliary steam supply. This supply is used exclusively for the air heater soot blowers when the air heater is required to be cleaned during start up of the boiler. The auxiliary supply is connected downstream of the section valve, which remains closed when this source is being used. The drainage pipe work is isolated by a number of motorized drain valves.

Location of Soot Blowers:

The blowers are divided into groups, numbered from 1 to 9. Group 1 to 4 comprises of wall blowers and group 5 to 9 comprises of long retractable soot blowers. The soot blowers within each group are listed in pairs, with an odd numbered soot blower paired with an even numbered soot blower.

Wall Blower:

Presently there are 88 wall blowers, which are numbered from 1 to 88. All wall blowers are located at four levels, so grouped as group-1, group-2, group-3 & group-4, of first pass of boiler each having 22 blowers. Group-1 is below furnace zone and other three are above furnace zone. Long Retractable Soot Blower:

LRSBs are kept in LHS and RHS of the boiler. Odd numbered LRSBs are kept in RHS and even numbered LRSBs are kept in LHS. With an exception of group 6, all soot blowers in a particular group are operated in synchronous pairs in an automatic sequence i.e. an odd numbered soot blower with an even numbered soot blowerLRSBs are mainly used to penetrate between the tube banks of the economizer, superheater etc.It consists of a fabricated box casing, steam lance tube, steam feed tube, steam isolating valve, traverse and rotary chains, etc

Whenever the signal to operate the soot blower is given from the control room, an electric motor drives the traverse reduction gearbox from its parked position, at a preset distance the steam isolating valve opens to allow steam to flow through the feed tube and out through the lance tube nozzles. The pattern of the steam jet from the nozzles is helical with a different pitch on the inward stroke to that on the outward stroke. At about mid-stroke a feed tube support rises to abut the feed tube preventing sagging.

Whenever the extent of the stroke has been reached, a reverse limit switch is actuated causing the main gearbox/lance tube to reverse and return to its parked position engaging the stop limit ready for the next operation, having re-set the feed tube support mechanism and isolated the steam isolating valve.

An air scavenge valve is fitted to the steam isolating valve which prevents the ingress of boiler gases into the valve chest and provides a positive air flow when the soot blower is in the parked position.

Air is also provided for the wall box to prevent gas leakage from the boiler.

SOOT BLOWER ARRANGEMENT

FRONT WALL

REAR WALL

123124

125126

WALL DESLAGGER

AIR HEATER BLOWER

LONG RETRACTABLE SOOT BLOWER

STEAM TO MECHANICAL POWER

INTRODUCTION

A thermal power plant is based upon the principle of conversion of heat energy into mechanical energy. For this conversion of energy a power plant requires a turbo machine. A turbo machine is a power producing thermodynamic machine. In order to function, a turbo machine requires a suitable working fluid, a source of high-grade energy and a sink for low-grade energy. In a thermal power plant water is used as a working fluid and it is converted into steam. A steam turbine is a device that converts heat energy of the steam coming from the boiler into the mechanical energy of shaft rotation. NTPC Rihand has four 500 MW units. Each unit has one steam turbine. For large turbo machines, multicylinder designs are used. The number of cylinder depends on the terminal conditions of steam and speed of rotation. The turbine with a number of cylinders on a single shaft is described as a Tandem compound machine. NTPC Rihand is a fossil-fired power station using a typical turbine of 500 MW output in each unit. Each of the two turbines consists of a single flow HP, a double flow IP and two double flow LP turbines (cylinders) coupled to the generator shaft.

Each section of steam turbine consists of a rotor from which project several rows of closely spaced blades. Between each row of moving blades there is a row of fixed guide vanes (blades) that projects inward from circumferential housing. The vanes are carefully shaped to direct the flow of steam against the moving blades at some angle and at a high velocity that will maximize the energy conversion. The cross-sectional diameter of a turbine section increases continuously and so the steam expands. In this process the temperature and pressure of the steam decreases and volume increases continuously. The LP section has the largest cross-sectional diameter, whereas the HP has smallest cross-sectional diameter. Separate stop valves and governing valves are provided to control the steam inlet and outlet from the turbines.

The main auxiliaries of a steam turbine are the rotor, blades, guide vanes, casing, bearings and couplings, bolting, steam chests, valves and critical piping.

INTRODUCTION TO 500 MW POWER PLANT

BOILER:

The boiler is a radient, controlled circulation drum, dry bottom type unit. The boiler units are designed for the following terminal conditions (mcr):

Evaporation a)SH outlet : 1,725 t/hr

b)RH outlet: 1,530 t/hr

Working pressure after stop valve:178 kg/cm-2

Steam Temperature at SH outlet :540 deg C

Steam Temperature at RH Inlet :344.1 deg C

Steam Temperature at RH Inlet :45.85 kg/cm-2

Steam Temperature at RH Inlet :43.36 kg/cm-2

Feed Water Temperature at ECO Inlet:256 deg C

Furnace Design Pressure : +/- 660 mm wc(g)

The boilers are of single furnace design,circulating pumps to provide assisted circulation.

Each Boiler corner is fitted with tilting tangential burner boxes comprising for high energy ignitors, four light-up heavy oil fired burners and eight pulverized coal burners. The angle of tilt from the horizontal is about 30 deg to +30 deg. Feed water to the boiler passes through HP feed heaters into the economizer and then to the steam drum from where it flows into the suction manifold and furnace wall circuits via the three boiler circulating pumps, returning to the steam drum as a water/steam mixture.This mixture is separated into three stages and first two stages are incorporated into the turbo separators and the final stage takes place at the top of the drum just before the steam enters the connecting tubes comprising of first stage superheating . within the steam circuit there are a further four stages of superheating, making five in total. There are also three stages of reheat. superheater temperature control is provided by spray attemperation situated in the connecting link between the superheater low temperature pendant outlet header and the superheater division panel inlet headers. Pulverized coal system The system for direct firing of pulverized coal utilizes bowl mills to pulverize the coal and a tilting tangential firing system to admit the pulverized coal together with the air required for combustion(secondary air) to the furnace.

As crushed coal is fed to each pulveriser by the feeder,primary air supplied from the primary air fans which dries the coal as it is being pulverized the transports the pulverized coal through the coal piping system to the coal nozzles in the wind box assemblies.

The pulverized coal and air discharged from the coal nozzles is directed towards the center of the furnace to form firing circle.

Primary air system

The primary air draught plant supplies hot air to the coal mills to dry and convey pulverized coal to the burners.

The P.A system comprises two P.A. fans,two steam coil air preheaters (SCAPH) and two regenerative air preheaters.Each fans, which is of sufficient rating to support 60% MCR load,discharges through a SCAPH into a common bus duct that has four outlets,two directing air into the primary air preheater for heating, two direct cold air straight to the pulverized mills.

Secondary air system

The secondary air draught plant supplies the balance of air required for pulverized coal combustion, air for fuel oil combustion, and overfire air to minimize the production of nitrous oxide(NOX).

The secondary air system,comprises two forced draft (F.D.) fans, two steam coil air preheaters

(SCAPH) and two regenerative type secondary air preheaters.

Flue gas handing system

The flue gas handling plant draws the hot flue gases from the furnace and discharges, them to atmosphere through the chimney. During its passage to the chimney. During its passage to the chimney, flue gas is passed through a feed water economizer and four regenerative air preheaters to improve boiler efficieny, and through four electrostatic precipitators to keep dust emission from chimney within prescribed limits.

soot blowing system

On load,gas side cleaning of boiler tubes and regenerative air heaters is achieved using 126 electronically controlled soot blowers which are disposed around the plant as follows:

88 - furnace wall blowers : steam

34 - long retractable soot blowers : steam

4 - Air heater soot blowers primary

and secondary air heaters : steam

TURBINE

GENERAL DESCRIPTION

The turbine is a reaction, condensing type, tandem compound with throttle governing and regenerative system of feed water heating. It is coupled to directly to the generator. The turbine is suitable for sliding pressure operation to avoid throttling losses at partial loads. The turbine is a single shaft machine with separate HP, IP and LP turbines. HP turbine is a single flow cylinder where as IP and LP are double flow cylinders. Steam flow to HP Turbine is controlled by four combined main stop and control valves by a simple throttle governing system. On the two exhaust lines of HP turbine, swing check valves are provided which prevent hot steam from the reheater flowing back into the HP turbine. The hot reheat steam is admitted to the IP turbine through the four combined stop and control valves. IP exhaust is connected to the LP turbine by cross over pipes without valve sat diametrically opposite points.

HP TURBINEThe outer casing of the HP turbine is of the barrel type and has neither axial nor a radial flange. The guide blade carrier is axially split and kinematic ally supported. The space between the outer casing and the inner casing is fed from admission steam to HP turbine. This steam is drained through HP casing drain during start up which promotes quicker heating of inner casing which results in lesser problems of differential expansion. The inner casing is attached in the horizontal and vertical planes in the barrel casing so that it can freely expand radially in all directions and axially from a fixed point. The HP turbine is provided with a balance piston in the admission side to counter act the axial thrust caused by steam forces. HP turbine is provided with 18 stages of reaction blades.

IP TURBINE

It is of double flow construction and consists of two casings. Both are axially split and inner casing kinematically supported and carries the guide blades. The inner casing is attached to the outer casing in such a manner as to be free to expand axially from a fixed point and radially in all directions. IP turbine is having 14 reaction stages per flow. Extraction steam to high pressure heater no.5 and deaerator are taken from IP turbine and the IP-exhaust respectively.

LP TURBINE

The casing of the double flow LP cylinder is of three-shell design. The shells are axially split and of rigid welded construction. The inner shell taking the first rows of the guides blades, is attached kinematically in the middle shell. Independent of the outer shell, the middle shell, is supported at four points on longitudinal beams. Two rings carrying the last guide blade rows are also attached to the middle shell. LP turbine is provided with 6 reaction stages/flow.

TURBINE SYSTEM

HP STEAM SYSTEMThe high pressure and temperature steam is piped from the boiler stop valves to the steam inlet ends via two steam legs or steam pipes. These two steam pipes are further branched into four sub-branches. These branches end-up into four inlet valves of four horizontally arranged steam chests, two on each side of the HP cylinder. Each of the steam chest contains a HP emergency stop valve and a HP governing valve arranged co-axially. The HP stop valve is a crucial valve and is used only during emergency conditions. Steam leaves the governor valves through branch pipes to the top and bottom inlet tee pieces on the HP cylinder. The steam chest assemblies are mounted on the foundations close to the HP cylinder. Each stop and governor valve is opened by a hydraulic operating gear and closed by two coil springs.

IP STEAM SYSTEMThe steam expands in the HP turbine and is fed into the reheater section of the boiler. From reheater section of the boiler the steam is piped into the steam inlet ends of the intermediate pressure turbine through four inlet pipes.

There exists four steam chests two on each side of the IP cylinder. Each steam chests contains an IP intercept stop valve and an IP governing valve arranged

co-axially. Steam leaves the governor valves through branch pipes to the top and bottom tee pieces on the IP cylinder.

The opening and closing mechanism of the valves of the IP turbine is similar to that of the HP stop valves and governing valves.

LP STEAM SYSTEM

IP steam after expansion is exhausted through four stub pipes, two at each end at the top half of the IP cylinder, and is ducted to the two LP cylinders by IP/LP crossover pipes. The volume of steam at the IP outlet is enormous and to handle such an enormous volume of steam two LP turbines are provided.

The steam from each of the LP turbines after expansion is fed into the condensers mounted below the LP turbines. In the condensers the steam outlet from the LP turbines is condensed to give condensate, which further undergoes a cyclic process to give feed water for the boiler input.

Here, under slung condensers with 800 inclined tubes are used, using a connecting duct between the turbine outlet flange and condenser inlet flange. As the condenser tubes are normally much longer than the width of the turbine casing, so this duct is trapezoidal in shape.

The heat energy carried by the steam is converted into mechanical power, which results into the shaft rotation. The turbine shaft is coupled with the generator shaft.

This rotary motion of the generator shaft is converted into electrical power by the generator. The power output from the generator is at low voltage level of 20 kV`. This low voltage is boosted to high voltage level of 400 kV to reduce the transmission losses

.

TURBINE AUXILIARIES

Turbine casing

Steam turbine casings are the massive steel casings that encase the internal stationary and rotating components of the turbine. Turbine casings have two critical functions:

a) Containing the steam pressure and

b) Maintaining support and alignment of the internal components.

There are generally two casings:

i) Outer casing

The outer casing is divided on the horizontal centerline into top and bottom half casings. Both the halves are bolted together with bolts and cap nuts at their horizontal joint flanges.

ii) Inner casing

The inner casing is subjected to the highest temperatures and pressures, is also divided at the horizontal centerline into top and bottom half casings, which are bolted together in a similar manner at their horizontal joint flanges.

The inner casing is supported within the outer casing in such a manner as to maintain proper alignment and allow thermal expansion. Cracking of the casing can lead to steam leaks and, in extreme situations, to bursting. Cracks in casings are typically located at the inlets of HP and IP turbine sections, where the local thermal stresses are higher.

Turbine BladesBlades are the single most costly parts of the turbine. The function of the turbine blades is to convert the available heat energy carried by the steam into mechanical energy. Blades fitted in the stationary part are called guide blades or nozzles and those fitted in the rotor are called moving blades.

Steam chests and valvesEach of the HP, IP, LP cylinders of the turbine has four steam chests two on each side of the turbine. Steam chests are the pressure vessels or steam carrying spaces, which performs the function of both carrying as well as transferring the steam at high pressure and high temperature via valves into the turbine cylinders. These steam chests comprises of a stop valve and a governing valve. The steam from the superheaters outlets is admitted to the HP steam chests via four HP steam mains. The steam from the steam chests is admitted to the HP turbine steam inlet pipe through the emergency stop valves and the associated governing valves which are housed in steam chests. After expansion in the HP turbine the steam is fed into the reheater and then to IP steam chests, through HP turbine steam outlet pipe. The steam from IP steam chests is fed into the IP turbine via IP stop valve and IP governing valves and then to LP steam chests.

Stop valvesThe stop valves are emergency valves placed in the steam chests and are provided in the main steam line. The purpose of the emergency stop valves is to cut off the steam supply during periods of shutdown and to provide prompt interruption of steam flow through the turbine cylinders in an emergency trip.

Normally, these valves are kept open fully but during emergencies these valves are fully closed (100%). During such conditions, the plant is producing no power because no input is given to the turbine.Governing valvesThe governing valves are also situated in the steam chests. These are the control valves i.e. they provide accurate control of the steam flow rate into the turbine, thus controlling the generator load when the machine is synchronized to the grid.

HP governing valves are the most important valves. They controls the amount of power produced by the plant or unit. For a unit producing 500 MW power, the HP governing valve opening is kept as 40-45% of the total valve opening.

Turbine Oil System:

OIL SUPPLY SYSTEM

The oil supply system fulfills the following functions:

a) Lubricating and cooling the bearings.

b) Driving the hydraulic turning gear during interruptions to operation, on start up and shut down [1].

c) Jacking up the shaft at low speeds (turning gear operation, start up and shut down) [1,2].

OILSYSTEM

Under normal operating conditions, the main oil pump (1) situated in the bearing pedestal and coupled directly to the turbine shaft draws oil from the main oil tank (26) and conveys it to the

pressure oil system.

The suction of the main oil pump is aided by two injectors (25). The injectors produce pressure at the suction connection to the main oil pump sufficient for all types of operation. This guarantees that the main oil pump takes over the safe supply of oil and cavitations that could occur due to greater suction heads are avoided. The amount of oil required for driving is extracted from the pressure oil circuit and adjusted by means of the throttle (31).

The oil for the turning gear (7) is also extracted from the pressure oil system. Oil is admitted to the nozzles by opening the shut off valve (33). The pressure oil is cooled in the oil coolers (19) and reduced to lubricating oil pressure in the throttle (18). The throttle is adjusted on the initial start-up. The amount of oil required for each bearing is adjusted on start up by means of the oil throttles (15).

FULL LOAD AUXILIARY OIL PUMPS

During turning gear operation and start up and shut down operation, one of the two three phase a.c., full load auxiliary oil pumps (22,24) supplies the pressure oil system and takes over the function of the main oil pump when this is not in operation because the turbine is running too slowly.

The full load submersible auxiliary oil pumps are situated on the oil tank (26) and draw in oil directly.

EMERGENCY OIL PUMP

When main and full load auxiliary oil pumps fail, the lubrication supplies oil directly to the lubricating oil line, by passing the oil cooler and thus preventing damage to the bearing shells.

OIL RETURN SYSTEM

The lubricating oil from the bearings is returned to the main oil tank via a header.

EXTRACTION OF OIL VAPOUR

The main oil tank is designed to the air tight. The extractors (28) produce a slight vacuum in the main oil tank and the bearing pedestals to draw off any oil vapour.

FILTERS

Oil for the thrust bearing is passed through the duplex oil filter which can be switched over and cleaned during operation.

MAIN OIL TANK

The main oil tank contains the oil necessary for the lubricating and cooling of the oil bearings and for the lifting device. It not only serves as a storage tank but also for deaerating the oil. The capacity of the tank is such that the full quantity of oil circulated not more than 8 times per hour. This result in a retention time of approx. 7 to 8 minutes from entry into the tank to suction by pumps. This time allows sedimentation and detainment of the oil.

Oil returning to the tank from the supply system first flow through a submerged inlet into riser section where the first stage deaeration takes place as the oil rises to the top of the oil tank. Oil overflows from the riser section through the oil strainer into the adjacent section of the tank where it is then drawn off on the opposite side by the suction pipe of the oil pumps. Main oil tank has the following mountings:

1. AC auxiliary oil pump

2. DC emergency oil pump

3. Shaft lift oil pumps

4. Oil injector

5. Oil vapour extractor

6. Oil level indicator

7. Sonar level limit switch

The main oil pump is situated in the front bearing pedestal and supplies the entire turbine with oil that is used for bearing lubrication, cooling the shaft journals and as primary and test oil. The main oil pump is driven direct from the turbine shaft via the coupling. These pump also convey oil in the suction branches

MAIN OIL PUMP WITH HYDRAULIC SPEED TRANSMITTER

The main oil pump is situated in the front bearing pedestal and supplies the entire turbine with oil that is used for bearing lubrication, cooling the shaft journals and as primary and test oil. The main oil pump is driven direct from the turbine shaft via the coupling. These pumps also convey oil in the suction branches of the main oil pump for oil injectors that maintain a steady suction flow to main oil pump.

Hydraulic speed transmitter operates on the same principle as centrifugal pump impeller. The variation of the pressure in the primary oil circuit due to a speed variation serves as a control impulse for the hydraulic speed governor. The hydraulic speed transmitter is supplied with control oil supplied from the control equipment rack. The suction of the pump is always flooded and hence maintains an uniform suction pressure.

AUXILIARY OIL PUMP

The auxiliary oil pump is a vertical one stage rotary pump with a radial impeller and spiral casing. It is fixed to the cover of the oil tank and submerges into the oil with the pump body. It is driven by an electric motor that is bolted to the cover plate of the main oil tank. The pump shaft has a sleeve bearing in the pump casing and a grooved ball bearing in the bearing yoke. The bearings are lubricated from the pressure chamber of the pump; the sleeve bearing via a bore in the casing; the grooved ball bearing via lube line.

DC BEARING OIL PUMPThis is a vertical, centrifugal submerged type and serves for lubrication and cooling of the bearing during emergency conditions when one of the other pump fails. This is driven by a D.C. motor.

SHAFT OIL PUMPThe lift oil pump is self-priming screw spindle pump with three spindles and internal bearings. The pump supplies the oil to lift the turbine rotor at low speeds.

OILVAPOUR EXHAUSTER

The function of oil vapour exhauster is to produce a slight negative pressure in the main oil tank and in the bearing casing and thus draw off the oil vapour.

OIL COOLER

Function of oil cooler is to cool the lubricating oil supplied to the bearing of the turbine. Oil cooler consists of the tube nest, the inner, outer shell and water boxes. The tube nest through which the cooling water flows is surrounded by the oil space formed by the outer shell. The oil is to be cooled enters the oil cooler and flows to the inner shell. This shell supports the large baffle plates, which are provided with an opening in the center. Between every two large plates there is a small intermediate plate is smaller in diameter than the inner shell and leaves an annular gap. This arrangement serves to achieve a cross flow pattern forcing the oil flowing to the outlet branch to flow through the middle of the large plates, while passing round the edge of the short ones.

Turbine gland sealing system:

HP-Turbine, IP-Turbine and LP-Turbine gland leak off are connected to seal steam header and vapour exhauster system. Initially gland steam requirement for all the three cylinders is met by supplying auxiliary steam to the seal steam header and the header pressure is maintained by the seal steam control valve. When the unit load is raised above 30 to 35% HP & IP-glands start supplying gland leak off steam to the header to the requirement of LP-glands. Beyond 40% load, no auxiliary steam is required. Once the self sealing steam takes place, then seal steam header pressure is maintained by opening the leak of steam control valve to the condenser.

Gland steam header is provided with a motor driven drain valve which helps to raise the gland steam temperature during start up. This drain valves normally remains open till the gland steam temperature to LP gland increases beyond 150.

CONDENSATE WATER SYSTEM:

THE PURPOSE OF THIS SYSTEM IS TO STORE AN ADEQUATE QUANTITY OF DEMINERALIZED WATER TO MEET THE MAKE UP REQUIREMENTS FOR NORMAL CYCLE FLUCTUATIONS AND FOR ABNORMAL OPERATING CONDITIONS WHEN SUPPLY OF DEMINERALIZED WATER IS INTERRUPTED.

This system will transfer condensate to and from storage tanks as to satisfy main cycle requirements. The main cycle flow and thermodynamic requirement is maintained by transporting the condensate collected in the condenser hotwell through various stages of feed water heating and other equipment to the deaerating feed water heater.

The condensate extraction pumps normally deliver the condensate through the three low pressure feedwater heaters, the deaerating feed water heater to the deaerating storage tank. The low pressure feed water heaters receive extraction steam as it passes through feedwater heater. The deaerating further preheats the condensate prior to its entry into the deaerating storage tank. The deaeratig feed water heater is warmed by extraction steam during the normal operation and auxiliary steam & cold reheat steam are utilized as the heat source during start up and shut down condition.

The normal make up to the condenser is supplied from demineralizing plant through the makeup pumps. Normally, on low level in the condenser hotwell, condensate will flow from the condensate storage tank to hotwell by static head in the tank and differential pressure due to the condenser vacuum, however, this flow will be inadequate, the condensate, transfer will supplement the flow. This makeup is sprayed into the steam space above the tube bundles.

The condenser hotwell is condensate collection vessel, integral with the condenser shell, and located in a pit below the ground floor. Condensate collected in the hotwell is pumped by 3*50 % condensate extraction pumps to the feed storage tank through feedwater placed in series. Two lines from hotwell, make a common header where from three lines are connected at the suction of three condensate extraction pumps.

The suction piping to the pumps is vented back to the condenser, to insure that the non-operating pumps stays completely flooded. These vent lines include manual valves on the vent for each pump.

A minimum flow (350 T/hr) recirculation line for each pump is provided, returning to the condenser via a flow control valve and a locked open shut off valve.

The shaft seals of these pumps are the water-injected type fed from a header to prevent the suction of air, particularly the pump that is not operating while the condenser is under vacuum.

Condensate Extraction Pump:

The function of Condensate extraction pumps is to pump out the condensate to the deaerator through gland steam cooler, and LP heaters. The steam from the LP cylinders exhausts into the condenser shells where it is constrained to flow across the water tubes, through which cooling water is circulated.

The steam condensed on the tubes drain to the bottom of the shell .The condensate is retained in the condenser shell bottom by means of the condenser level control valve. The water in a condenser provides a head of water for the condensate extraction pump to suppress cavitations in its suction impellers.

There are two 100% duty extraction pumps, one remains in duty and one remains stand by. With all the necessary instruments such as suction and discharge valve isolating and dump valves to insure efficient operation.

The thrust bearings in the driving motors have temperatures sensor, which can trip the motors automatically.

The pump discharge the condensate via the gland steam condenser and the condensate polishing plant to the LP feed heating system.

Air Extraction Pump:

The function of the air extraction pump is to raise and maintain the vacuum conditions in the turbine main condensers, and to remove air and other non-condensable gases vented to the condenser from various parts of the turbine and feedwater heating system.

Gland steam condenser

An automatic turbine gland sealing system is used to prevent the escape of steam into the turbine hall, where it would condense on the walls and the plant.

It also prevents the ingress of air at the rotor ends of all the turbine cylinders. This is done by using the gland steam condenser.

The function of the gland steam condenser is to maintain a sub-atmospheric pressure at the outermost leak-off belt of the glands and thereby prevent the leakage of steam from the glands into the turbine hall.The gland steam condenser condenses the steam from the steam/ air mixture drawn from the outer pocket of the gland. This heat given by the steam is absorbed by the feedwater. Condensate formed in the shell drains to the gland steam condenser drain tank. This is again recirculated back to the condensers.

Drain Flash Condenser

The drain flash condenser receives heater drip or drain water from LP heaters. When the drain water enters the flash condenser through the disperses, steam is flashed off. This steam is drawn into the tube nest where it condenses and cascade to the bottom of the shell. The resulting drain water is delivered to the main condenser.

The water side of the DFC is of the conventional to pass surface type in which the feed water flows through the tube and steam passes over them. The vessel comprises a tube nest, a shell, an inlet and an outlet water box. The water box has a dished end and incorporates the feed water inlet and outlet connections. A flanged branch on the water box is fitted with a davit type hinged cover to give access to the tube bores.

A center plate and an end plate fitted with two stiffeners form a diversion plate, which separates the incoming, and outgoing flows .The end plate carries a cover, which is located and bolted between the stiffening ribs.

The DFC is mounted on steel work on four attachment points at the water box bonded to upper and lower mild steel bearing plates.

Boiler Feed Pumps:

Boiler feed pumps are an important part of any boiler operation. They control the amount of water fed to the boiler and the manner in which it is fed.

Centrifugal - ContinuousTurbine - Intermittent

In order to properly select a boiler feed pump five key points must be considered:

Will the pumps operation be continuous or intermittent?This is an operational question and is often answered by the type of level control found on the boiler that the pump will be servicing. As a general rule of thumb, boilers with a capacity of 10,000 lbs./hr. or less utilize a float type switch that starts and stops the boiler feed pump to satisfy a predetermined water level within the boiler. This is a classic intermittent operation.Boilers with capacities exceeding 10,000 lb./hr. typically employs a modulating feed water regulator and will continuously feed water to the boiler at various rates depending upon the water level in the boiler.By knowing which operation you are to satisfy, you can determine which pump design is best suited for your application. As a general rule of thumb a turbine pump is used in an on-off situation and a centrifugal pump is used for continuous operation. But remember, this is a general rule and is some cases a centrifugal could be used for an on-off application and a turbine for continuous.

What is the temperature of the water being pumped?It is also important to know the temperature of water you intend to pump. Most pumps can usually handle 215F to 230F, other pumps are available that can handle higher temperatures by using external water-cooling. Keep in mind that a deaerator pump must be able to handle higher temperatures because they operate at a 5 psi or 227F.

What is the required capacity?

How much water you intend to pump is dependent upon the evaporation rate of the boiler the pump will service. A safe figure for an on-off application would be 2 times the evaporation rate of the boiler. With a modulating level control, a factor of 1.3 times the evaporation rate plus recirculation is recommended.

What is the desired discharge pressure?When you pump directly into the boiler you will need to overcome the pressure in the boiler as well as any piping losses. You can chose the right pump by looking at the pump curves to determine which will accomplish this task. Should you have a modulating valve in the discharge line, the minimum you will need to add to the boiler operating pressure will be 20 to 25 lbs. Make sure that the pump can handle the pressure along with the flow rate needed. With an on-off level control the pumps should be designed for the relief valve pressure.

What is the NPSH or net positive suction head required?This is the last piece of information that you will need. This is the minimum absolute pressure at the suction nozzle at which the pump can operate. To avoid pump cavitation, the NPSHA of the system must be greater than the NPSHR of the pump. In other words, the available NPSH must be higher than the required. We have always sized our deaerator stands to be two feet higher than the NPSH needed for the pump selection. Remember, the water level in the storage tank adds to the safety margin.

TDBFP

Turbine driven BFP uses turbine of 14 stages connected to condenser. Turbine is coupled with main pump having engage/disengage unit called Power pack unit using oil pressure. Between turbine and booster pump gear assembly is there.

Various system of TDBFP are discussed below:

1) Lube Oil System

Lube oil system of both TDBFPS are provided with one Main Oil Tank each in which oil level is separately maintained. It has two AC AOPs, one JOP AC and one DC AOP connected to tank. Lube oil pressure is maintained at 3.0-kg/sq. cm is called control oil which is used as governing oil.

Lube oil after passing through coolers is led to various bearing of TDBFP system. DC AOP discharge oil, which is, used on failure of AC AOPs bypass the coolers. During barring of TDBFP same lube oil at pr. 4.0-kg/ sq. cm is used as power fluid in barring gear impellers. In the lube oil pressure by changing the recirculation flow.

2) Seal/ Injection System

Mechanical seals are provided on BP side for which continuous cooling is done by CEP water. For BFP side constant seal injection pressure around 18-kg/sq. cm is maintained with the help of control valve. Filters are also provided in this line. From this line small pipe provides water in the exhaust steam as exhaust load spray. Seals drain is collected as clean drain into drain tank and dirty drain flows into common drain header.

3) Steam System

For TDBFP there is three sources for steam namely 1. Auxiliary steam 2. Cold reheat line 3. IP-LP cross over steam. Extraction steam parameters are maintained at 4.0 kg/sq cm. & 300 degree centigrade. During cold start when CRH or IP-LP steam is not available, Aux. Steamis used for rolling of TDBFP for initial boiler filling. Once steam is insufficient for increasing the speed beyond 3500 rpm, CRH steam is automatically cut off.

4) Feed Water System

Water from deaerator is taken through two suction into booster pump and then fed into BFP suction. Drains & vents are provided in FW system for initial charging and venting of BFP during rolling.

5) Gland Sealing of Turbine

Downstream steam from main turbine gland sealing is used for TDBFP sealing. Before opening of exhaust valve gland sealing should be done as this is line is connected to condenser.

6) Governing System

For having the required flow through BFP, speed of turbine has to be adjusted and for what we need perfectly efficient governing system. Governing system is using the control oil at 9 kg/sq cm. which in turn depending on the position of starting device and speeder gear, will develop secondary oil and auxiliary secondary oil pressure for operating the 4MCVs and one ACV thereby adjusting the steam flow speed change is effected. Governing is achieved with the help of two governors namely hydraulic and Electrohydraulic governors having EHG control and HG follows it up.

BOOSTER PUMP:

Each boiler feed pump is provided with a booster pump in its suction line which is driven by the main motor of the boiler feed pump. One of the major damages which may occur to a boiler feed pump is from cavitation or vapour bounding at the pump suction due to suction failure. Cavitation will occur when the suction pressure at the pump suction is equal or very near to the vapour pressure of the liquid to be pumped at a particular feed water temperature. Therefore all the three feed pumps are provided with a main shaft driven booster suction line for obtaining a definite positive suction pressure. The boiler feed pump is coupled with its driving motor through hydraulic coupling. The hydraulic coupling serves the purpose of controlling the speed of feed pump for maintaining a definite delivery head and delivered quantity of the feed water as per the requirement of the boiler.

Deaerator:

Deaerators are used to remove oxygen from boiler feed-water. It performs the following functions:

It serves as a feedwater heater.

Feedwater is deaerated by the removal of non-condensable gases present in the feedwater and reduce the oxygen content to a level of about 0.007 ppm.

It acts as a buffer to the fluctuations of condensate feedwater flow that occur in service durations.

The tank height provides a hydrostatic head to satisfy the suction pressure requirements of the boiler feed pumps.

The presence of non-condensable gases in the feedwater causes the waterside corrosion or thinning of the boiler tube walls and this further leads to the rupture of walls by the internal fluid pressure.

Here steam injection-type deaerator is used to deaerate the feedwater. Feedwater from LP heaters is admitted to the deaerator tank and steam is bled from two sources, one from the HP turbine (which is known as cold reheat steam), and the other from the IP turbine exhaust. When this steam and feedwater come in direct contact with incoming water is heated and also gets deaerated.

Steam initially enters the deaerator through a special, stainless steel Jet Spray atomizing valve. This valve is designed to fully atomize and provide complete deaeration under all load conditions. This design insures that the purest steam comes in contact with the purest water. Once this high velocity steam mechanically shakes out the last traces of non-condensable gases from the water, it flows upward into the pre-heater area where it meets the incoming make-up water and pumped returns. This water continuously condenses the steam, which preheats and releases the non-condensable gases into the internal vent condenser where they are metered harmlessly to the atmosphere. This action causes more steam to be drawn into 6the system to complete the cycle.

From the bottom of the deaerator, feedwater is piped through three outlet pipes, each with a strainer, to the three boiler feed pumps and the used up steam is recirculated back to condenser.

The deaerator is mounted at a high-level with its centerline 34.65 m above the datum level.

The deaerating heater utilizes steam by spraying the incoming water into an atmosphere of steam in the preheater section (first stage). It then mixes this water with fresh incoming steam in the deaerators section (second stage).

In the first stage the water is heated to within 2 C of steam saturation temperature and virtually all of the oxygen and free carbon dioxide are removed. This is accomplished by spraying the water through self adjusting sprat valves which are designed to produce a uniform spray film under all conditions of load and consequently a constant temperature and uniform gas removal is obtained at this point.

From the first stage the preheated water containing minute traces of dissolved gases flows into the second stage. This section consists of either distributor or a several assemblies of trays.

Here the water is in intimate contact with an excess of fresh gas/free steam. The steam passes into this stage and it is mixed with the preheated water. Deaeration is accomplished at all the rates of flow if conditions are maintained in accordance with design criteria. Very little steam is condensed here as the incoming water has high temperature caused by the preheating. The steam then rises to the first stage and carries small traces of residual gases. In the first stage most of the steam is condensed and the remaining gases passes to the vent where the non-condensable gases flow to the atmosphere.

The water, which leaves the second stage, falls to the storage tank where it is stored for use. At this time the water is completely deaerated and is heated to the saturated steam temperature corresponding to the pressure within the vessel.

The condensate pressure just before the entry to deaerators shall be 3 psi more than the deaerator steam pressure.

FEEDWATER HEATERS

A feed water heater is a special form of a shell and tube heat exchanger designed for the application of recovering the heat from the turbine extraction steam by preheating the boiler feed water. Its principal parts are a channel and a tube sheet, tubes, and a shell. The tubes may be either bent tubes or a straight tubes. Feed water heaters may be defined as high-pressure heaters when they are located in the feed water circuit upstream from the high pressure feed water pump. Low-pressure feed water heaters are located upstream from the condensate pump, whichs takes its suction from the condenser hot well. Typically low pressure feed water heaters are designed for feed water pressures between 27 kg/cm sq. and 57 kg/cm sq., high pressure feed water heaters range from 112 kg/cm sq. to 335 kg/cm sq. for super critical boilers.

Each feed water heater bundle will contain from one to three separate heat transfer areas and zones. These are condensing, desuperheating, and sub-cooling zones. Economics og design will determine what combination of the three is provided in each heater.

A condensing zone is present in all feed water heaters. Large volumes of steam are condensed in this zone and most of the heat is transferred here.

The desuperheating zone is a separate heat exchanger contained within the heater shell. This zones purpose is to remove superheat present in the steam. Because of the high steam velocities employed, condensation within the desuperheating zone is undesirable.

The sub-cooling zone is another separate counter heat flow heat exchanger whose purpose is to sub-cooling incoming drains and steam condensate.

HEATER OPERATION:

Prior to opening the feed water valve, the channel start up vents are to be opened and remain open until all passages have been purged and feed water begins to discharge.

To remove air from the shell sides of a heater, which does not operate under vacuum, the shell start up vent valves should be opened prior to the admission of the steam to the feed water heater. The extraction lines must be free of all the condensate to prevent the damage to the heater internals by slug flow.when the drains outlet valve is opened, the shell start up vent valves are to be closed and the operating air vent valves are to opened. Continuous venting of air and other non-condensable is assured by keeping the shell operating vent valves open. On initial plant start up of the horizontal feed water heaters, having integral drain coolers, the liquid level is to be kept just below the high-level alarm point.

On initial plant start-up of horizontal feed water heaters, having integral drain coolers, the liquid lev