Safety moduleOverview of safety hazard:What is hazard?A hazard is any source of potential damage, harm or adverse health effects on something or someone under certain condition at work.How to behave inside the plant:

No loose clothing. Safe distance is to be maintained from rotating machine. Any electrical installation should not be touched. Safe distance should be kept from bare live terminals. Helmet is a must. Nobody should move in dark spaces. Nobody should move alone. Be aware from floor opening. Avoid moving on checker plate. Nobody should move over the trenches. Railing should not be leant upon. Moving into confined spaces(congested closed area with lower

concentration of oxygen, and containing mixture of toxic and explosive gases) for eg any flue duct, furnace, inside the condenser, cooling water duct, sewerage lines is to be avoided.

One should not move in regions where lifting work is being carried out.

One should not go into a unit which is under maintainance. One must avoid going near welding and gas cutting sparks and must

use safety glasses.

. SUPPLY OF RAW MATERIALS:

A)COAL:There are 3 sources of coal supply as: Lalmatia situated 85 Kms away from the main plant. Imported coal from Indonesia. Indian railways who supply coal from different mines across the

country.B) WATER:

Feedwater: Collected from feeder canal cut across the river Ganga.

Cooling Water: Collected from the same source.

C) AIR: atmospheric air is used.

D) OIL: HFO is supplied by IOCL.

ASH DISPOSAL: For the quality of coal used here, approximately 80% Fly Ash and 20% bottom ash is produced. Ash disposal is a major issue in coal fired power plants.Raw water is mixed with ash to form slurry and pumped out through series pumps( 3 for Fly ash and 2 for Bottom ash ) into ash dyke.Fly ash dyke is situated 12 Kms away and Bottom ash dyke MALANCHA is situated 5 Kms away from the main plant.

DIFFERENT CYCLES INVOLVED IN THE ENTIRE OPERATION:

FUEL CIRCUIT : Coal is brought into CHP with the help of Wagon tipplers or

track hoppers and stored in the bunker. From bunker coal is sent to feeder via conveyer. The purpose of feeder is to vary the amount of coal as per

load requirement. From feeder coal is taken to mill where it is pulverized. Primary air is sucked from atmosphere through PA fan and

takes pulverized coal to the furnace.

Fig1: Coal circuit2) AIR & FLUE GAS CIRCUIT:

Secondary air is sucked from atm. with the help of FD fan(Forced Draught) for combustion.A fire ball is generated inside the boiler.The temp. of fire ball is 1000-1100 deg. C.

Flue gas is generated due to combustion. Heat is extracted from flue gas to heat the steam in platen SH,

reheater,final SH, primary SH , economiser and to heat air in air preheater.

Fly ash particles are collected from flue gas at ESP(Electrostatic Precipitator) and flue gas is released to atm.

with the help of ID fan(Induced Draught) which is situated at the base of chimney.

Fig2: Air & Flue gas circuit

3) STEAM, CONDENSATE & FEED WATER CYCLE: In 200MW unit of the plant the boiler drum is located at the

height of 54m. Inner wall of the boiler made up of water tubes which originate

from bottom ring header situated 7m from ground level. DM water picks up heat from fire ball and goes up as a

mixture of steam and water.Then it reaches drum .As steam being lighter it goes upward.Water comes down through DOWNCOMER .In 200MW unit no. of downcomers is 6 and natural circulation of steam and water takes place.But in 500MW unit the boiler drum being at a height of 72m forced circulation is employed by BCP(Boiler Circulation Pump) through 10 downcomers.

Steam is separated from water in the boiler drum with the help of CYCLONE SEPARATOR & SECONDARY SEPARATOR(corrugated sheets) and it follows the circuit shown below.

Fig 3: Steam,Condensate & Feed Water cycle

4) COOLING WATER CIRCUIT: Raw water is taken from the feeder canal through CW pump. Raw water is passed through TRASH RACKS and TRAVELLING

WATER SCREENS to remove impurities(animal bodies, planktons etc) of different sizes.

Raw water is stored at the water box at the inlet of condenser and it flows through the innumerable number of condenser tubes. In stage I there are 13000 such tubes. The water comes at the outlet and is stored at the outlet water box.

The condensate is stored in the HOTWELL at a temperature of 45 deg C.

In STAGE-I and STAGE-II,after using in the condenser the hot water is returned to the feeder canal, hence it is an open cycle system.

In STAGE III, Cooling tower is used. After using the water in the condenser it is recirculated through cooling tower, where the hot water is cooled and reused. This is a closed cycle system and causes less harm to the environment.

Fig4: Cooling Water circuitBasic Description of Flow of Coal:

(1)CHP (Coal Handling Plant): The input coal is unloaded at this portion, which passes it to the next block crusher house.

(2)Crusher house: crusher house crush the unloaded coal coming from the CHP in small size.

(3)Coal Bunker: Coalbunker stores the crushed coal and passes it to the Feeder.

(4)RC Feeder: It is the raw coal feeder. The coal, comes from the crusher house is still of big size. The feeder controls the quantity of the input coal to the boiler.

(5)Mill/pulveriser: Mill contains three rollers. It crushes it to powder finely. Primary air takes the pulverized coal from the mill to the boiler.

(6)Furnace : At furnace pulverized coal is combusted with the help of secondary air.

Brief description of system components:Main components of the power plant are:

1) Boiler: In Farakka NTPC, corner fired boilers are used. In stage I,

there are 6 mills situated from a height of 18m to 27m. FIRING FLOORS of mill A is at a height of 18m and that of mill F is at 27m. Rest of the firing floors are situated in between. HFO OIL GUNS are situated at heights of AB, CD and EF.

Purpose of oil gun: a) during boiler start up , b) emergency purpose.

Fig 6: HFO Flow System

The working oil supplied by IOCL is stored in HFO tank 2. The oil in this tank is replenished by oil in tanks 1 and 3. The heat lost due to transportation is compensated by HEATER. HFO Circulation takes place all the time so that temperature of oil is maintained at 120 deg C when plant is running.

In case of shut down of any mill, the coal particles on the corresponding floor may not get sufficient ignition energy and thereby the fireball distorts and subsequently two separate balls are formed on either side of that mill. In that case HFO injection makes up that gap and stabilizes the fire ball.

The coal is composed of 4 parts – fixed carbon(FC),Volatile material(VM),Moisture & ash. The VM supplies the initial ignition

energy for the complete combustion of FC. If the VM content in the coal used is less the coal particles have to travel a larger path to attain the required ignition energy and hence the fireball is formed at greater height.

Fig7: Isometric View of Boiler schematic

Fig8: Top view of firing floor corresponding to mill A

It is critical to maintain the level of water in boiler drum.

Case 1: level of water goes too high: water may go into the steam circuit. When water hits the turbine blades, PITTING may occur. (the high velocity water particles may produce indentations in the turbine blades)Case 2: level of water goes low: If it is too low, the boiler drum base being close to the fire ball, its shape gets deformed. Also low water level causes water in the drum to get evaporated at a faster rate than the rate at which water is returned from hotwell to the drum There are mainly three types of heater coils in boiler.

(1) Economiser (2) Re heater (3) Super heater

Economizer :

Economizer is a device in which the feed water is heated before it enters into a boiler. The heat is being taken from the waste flue gas of the boiler. It ensures economy of fuel. Hence it is called economizer. The economizer is placed at the second pass of boiler. The flue gases of the boiler furnace, after working inside the boiler, flows through it before passing into the chimney.

Coal bunker-

These are in process storage silos used for storing crushed coal from the coal handling system. Generally, these are made up of welded steel plates. Normally there are six such bunkers supplying coal of the corresponding mills. These are located on the top of the mills so as to aid in gravity feeding of coal.

P. A. Fan -

The primary air fans (2 per unit-50% capacity each) are designed to suck primary air from atmosphere. Temperature of PA is increased to 300 deg C after passing through Air Pre-heater. These fans are located at ‘0’M level near the boiler.

Burners-

As evident from the name itself, these are used for burning pulverized coal. Every unit has a set of such burners located at different elevations of the furnace.

F. D. Fan-

These forced draught fans (2 per unit-50% capacity each) are designed for handling secondary air for the boiler. These fans are located at ‘0’M level near the P.A Fan.

Electrostatic Precipitator- Air Pre-heater -

Air pre heater transfer the heat from the flue gas to cold primary and/or secondary air by extracting heat from waste flue gas. These are located in the secondary pass of the furnace at a height of around ‘16’M level. Each 200MW unit is provided with two such air pre-heaters.

ID Fans-

These are two induced draught fans per boiler located between the ESP & chimney. These fans are used for maintaining pressure inside the boiler by releasing flue gas through chimney.

Chimney-

These are tall RCC structures with single/multiple flue ducts (one flue duct per 200MW unit). The heights of these chimneys vary, depending upon the location considerations; anywhere between 150m to 220m.

Steam Circulation System Re-heaters-

This is the part of the boiler which receives the steam back from the turbine after it has given up some of its heat energy in the high pressure section of the turbine. It raises the temperature of this steam, usually to its original value, for further expansion in the turbine. The purpose of this reheating is to add energy to the partially used steam. The construction & arrangement is similar to super heater. It has two sections- hot & cold Reheat sections. Due to resistance of flow through the reheat section, the hot reheat steam is at lower pressure compared to the cold reheat steam.

water treatment plant and storage

USE OF MAKE UP WATER:Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow down and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system.

Boiler feed pump-

It takes water input from the deaeretor at a very low pressure and pumps it to the boiler drum at a very high pressure (190Kg).

Condenser- Ejectors-

There are two 100%capacity ejectors of the steam ejecting type. The purpose of the ejector is to evacuate air and other non-condensing gases from the condensers and thus maintain the vacuum in the condensers.

Condensate Extraction Pump-

It extracts the condensate from the hot well of the condenser and pumps it to the de-aerator.

L. P. Heaters-

Turbine has been provided with non-controlled extractions which are utilized for heating the condensate, from turbine bleed steam. There are 4 low pressure heaters in which last four extractions are used.L.P.Heater-1 has two parts LPH-1A and LPH-1B located in the upper parts of the condenser A & B respectively. These are of the horizontal type with shell and tube construction. L.P.H 2,3,4 are of similar construction and they are mounted in row at 5M level.

Deaerator-

This is used to remove oxygen. Due to the presence of certain gases like oxygen, carbon-di-oxide, ammonia, etc. in water then it is considered harmful because of their corrosive effect on metals, particularly at elevated temperatures. The boiler feed water should be free from all dissolved gases. This can be achieved by embodying into the boiler feed system a de-aerating system, whose function is to remove dissolved gases from the feed water by mechanical means.

H. P. Heaters-

These are regenerative feed water heaters operating at high pressures and located at the side of turbine. These are connected in series on feed-water side & by such arrangement feed-water enters the HP heaters. The steam is supplied to these heaters from the bleed point of the turbine through motor operated valves. These have a group bypass protection on the feed waterside. Here the feed water flows through the tube spirals & is heated by steam around the tubes in the shall of heaters. These heaters are cylindrical vessels with welded dished ends & with integrated, de-super heating, condensing, & & sub cooling sections. This

design offers the advantage to optimize the arrangement of piping & the location of the heaters at power station.

2) TURBINE:

Fig 9: Turbine cycle

Steam after working in the HP turbine temperature is reduced from

540 deg to 340 deg C. Pressure is also substantially reduced. The line of flow of steam from HP turbine is COLD REHEAT LINE or CRH LINE.

CRH line again goes to the boiler to be reheated at 540 deg C by the heat of the flue gas. This line is known as the HOT REHEAT LINE or HRH LINE. HRH Line goes to the IP Turbine. Exhaust steam from IP Turbine goes to the LP Turbine.

Exhaust of LP Turbine (80 deg C) goes to CONDENSER. COOLING WATER is circulated to cool down the steam and the condensate drains into the HOTWELL at 35 deg C.

From HOTWELL the condensate is sent back to boiler drum through different processes and reused.

The pressure of MS line is ~160 Kg/sqcm, that of HRH line is ~33-34 Kg/sq cm and CRH Line pressure is ~34 Kg/sqcm.

With the help of modulation of HP Control valves (HPCV) and IPCV alternator frequency is kept constant by controlling the amount of inlet steam.

STOP VALVE is used to stop the entry of steam into the turbine.

Turbine Section

Turbine Lub. Oil System

This consists of Main Oil Pump (MOP), Starting Oil Pump (SOP), AC stand by oil pumps and emergency DC oil pump and jacking oil pump (JOP) (one each per unit).

Emergency stop valves and control valves

Turbine is equipped with emergency stop valves to cut off steam supply and with control valves regulate steam supply. Emergency stop valves (ESV) are provided in the mainsteam line and interceptor valves are provided in the hot reheat line. Emergency stop valves are actuated by servomotor controlled by the protection system. Control valves are actuated by the governing system through servomotors to regulate steam supply as required by the load.

Generator

The main shaft of the turbine is connected to the rotor of the main generator. The shaft rotates the rotor, which cuts the flux of the armature coil, and as a result an induced emf is produced.

Compressed Air System

There are two types of compressed air system namely Instrument air system or control air system & Station air system.