Ntpc (national thermal power corporation) sipat mechanical vocational training report 2 haxxo24 i~i

Vocational Training Project Report

On

“Thermal Power Plants”

National Thermal Power Corporation, SIPAT(Chhattisgarh)

(Submitted on completion of vocational training at NTPC, Sipat)

Project Guide: Submitted By:

Mr. U.R.Verma, Ritesh Patnaik,

Deputy General Manager, B.Tech (4th semester),

Boiler Maintenance Department. IIT Kharagpur.

DECLARATION

I hereby declare that this project is being submitted in fulfillment of the VOCATIONAL

TRAINING PROGRAMME in NTPC Sipat, and is the result of self done work carried out

by me under the guidance of various Engineers and other officers.

I further declare that to my knowledge, the structure and content of this project are original

and have not been submitted before for any purpose.

SUBMITTED BY:

RITESH PATNAIK,

ROLL NO:10ME10074,

B.TECH. (4TH SEMESTER),

INDIAN INSTITUTE OF TECHNOLOGY,

KHARAGPUR, WEST BENGAL.

CERTIFICATE

This is to certify that Mr. Ritesh Patnaik of Indian Institute of

Technology, Kharagpur has undergone Vocational Training for a period of 30

days from 21.05.2012 to 18.06.2012 at Sipat Super Thermal Power

Project, in the Boiler Maintenance Department, and has made the project

report under my guidance.

Project Guide:

Mr. U. R. Verma,

BMD Department.

ACKNOWLEDGEMENT

I am truly thankful to all the guides who imparted the lectures on various

subjects / topics and took me to the plant in a guided study visit along detailed

explaining about the plant and machinery.

I would give thanks to Mr. Pravin Patel & HR Dept. of NTPC Ltd, Sipat as they have given

me the chance of having this wonderful learning experience.

I am also indebted to respected Officers and Engineers:

1) Mr. U. R. Verma

2) Mr. Diptendu Mandal

3) Mr. Girish Choudhary

4) Mr. Rishabh Kapoor

5) Mr. Achal Deep Aggarwal

They went out of their way to provide me with as much information as they could, in spite of

the fact that they were laden with their own work. I can’t really express my feeling of

gratitude towards them.

About NTPC Sipat-

NTPC Sipat Super Thermal Power Project is a cynosure of power generation in India. It

is a priceless gem of mineral rich state of Chhattisgarh. Being in the vicinity of Bilaspur,

the second largest city of Chhattisgarh and at the vertex of transport arteries adds to its

majestic aura.

Total installed capacity of the Sipat Super Thermal Power Project is 2980 MW (Stage I-

660 X 3 & Stage II-500 X 2 MW).The present generation capacity of the project is 2320

MW(Stage I-660 X 2 & Stage II-500 X 2). Plant’s water requirement is catered by

Hasdeo right bank canal. It gets coal supply from Dipika mines of SECL, Korba. The coal

is transport via MGR system of total length 42 km.

Sipat Super Thermal Power Project has many accolades to its credits. Being a pioneer of

Super Critical Technology in India and transmission system of 765 kV for the first time

in India are the most unique attributes.

Addressing this most pressing need of the hour, i.e. our environment, Sipat Project has

100 meter wide peripheral green belt, submerged ash dykes and state-of-the art

technology for environment management.

BASIC POWER PLANT CYCLE:

RANKINE CYCLE

The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a

closed loop, which usually uses water. This cycle generates about 80% of all electric power

used throughout the world, including virtually all solar thermal, biomass, coal and nuclear

power plants. It is named after William John Macquorn Rankine, a Scottish polymath. The

Rankine cycle is the fundamental thermodynamic underpinning of the steam engine.

Overview of NTPC Sipat Super Thermal

Power Project:-

A thermal power station consists of all the equipments and a subsystem required to

produce electricity by using a steam generating boiler fired with fossil fuels or biofuels

to drive an electric generator. Some prefer to use the term ENERGY CENTER because

such facilities convert form of energy like nuclear energy, gravitational potential energy

or heat energy (derived from the combustion of fuel) into electrical energy.

The description of some of the components of the thermal power plant is as follows:

1. Cooling towers-

Cooling towers are eveporative coolers used for cooling water. Cooling tower uses the

concept of evaporation of water to reject heat from processes such by cooling the

circulaing water used in oil refineries, chemical plants, power plants, etc. Smaller

towers are normally factory built while larger ones are constructed on site. The

primary use of large, industrial cooling tower system is to remove the heat by

circulating the hot water used by the plants

The absorbed heat is rejected to the atmosphere by the evaporation of some of the

cooling water in mechanical forced – draft or induced draft towers or in natural draft

hyperbolic shaped cooling towers as seen at most nuclear power plants.

4 Nos Induced draft cooling towers with 10 fans each tower are installed at NTPC sipat

for the above said pupose.

2. Three phase transmission line& step- up transformer

Three phase electric power is a common method of electric power transmission. It is a

type of polyphase system mainly used for power motors and many other devices. In a

three phase system, three circuits reach their instantaneous peak values at different

times. Taking one conductor as reference, the other two conductors are delayed in time

by one-third and two-third of cycle of the electrical current. This delay between phases

has the effect of giving constant power over each cycle of the current and also makes it

impossible to produce a rotating magnetic field in an electric motor. At the power

station, an electric generator converts mechanical power into a set of electric currents

one from each electromagnetic coil or winding of the generator. The currents are

sinusoidal functions of time, all at the same frequency but offset in time to give different

phases. In a three phase system, the phases are spaced equally giving a phase

separation of one-third of one cycle. Generators output at a voltage that ranges from

hundreds of volts to 30,000 volts at the power station. Transformers step-up this

voltage for suitable transmission after numerous further conversions in the

transmission and distribution network, the power is finally transformed to standard

mains voltage i.e. the household voltage. This voltage transmitted may be in three

phase or in one phase only where we have the corresponding step-down transformer at

the receiving stage. The output of the transformer is usually star connected with the

standard mains voltage being the phase neutral voltage.

3. Electrical generator-

An electrical generator is a device that coverts mechanical energy to electrical energy,

using electromagnetic induction whereas electrical energy is converted to mechanical

energy with the help of electric motor. The source of mechanical energy may be a

rotating shaft of steam turbine engine. Turbines are made in variety of sizes ranging

from small 1 hp(0.75 kW) used as mechanical drives for pumps, compressors and other

shaft driven equipment to 2,000,000 hp(1,500,000 kW) turbines used to generate

electricity.

4. Steam turbine

A steam turbine is a mechanical device that extracts thermal energy from pressurized

steam, and converts it into rotary motion. Its modern manifestation was invented by Sir

Charles Parsons in 1884.

It has almost completely replaced the reciprocating piston steam engine primarily

because of its greater thermal efficiency and higher power-to-weight ratio. Because the

turbine generates rotary motion, it is particularly suited to be used to drive an

electrical generator – about 80% of all electricity generation in the world is by use of

steam turbines. The steam turbine is a form of heat engine that derives much of its

improvement in thermodynamic efficiency through the use of multiple stages in the

expansion of the steam, which results in a closer approach to the ideal reversible

process.

4. Steam condenser -

The condenser condenses the steam from the exhaust of the turbine into liquid to allow

it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam

is reduced and efficiency of the cycle increases. The surface condenser is a shell and

tube heat exchanger in which cooling water is circulated through the tubes. The

exhaust steam from the low pressure turbine enters the shell where it is cooled and

converted to condensate (water) by flowing over the tubes as shown in the adjacent

diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for

continuous removal of air and gases from the steam side to maintain vacuum.

5. Boiler Feed Pump-

A Boiler Feed Pump is a specific type of pump used to pump water into steam boiler.

The water may be freshly supplied or retuning condensation of steam produced by the

boiler. These pumps are normally high pressure units that use suction from a

condensate return system and can be of centrifugal pump type or positive displacement

type. Construction and Operation feed water pumps range from sizes upto many

horsepower and the electric motor is usually separated from the pump body by some

form of mechanical coupling. Large industrial condensate pumps may also serve as the

feed water pump. In either case, to force water into the boiler, the pump must generate

sufficient pressure to overcome the steam pressure developed by the boiler. This is usually

accomplished through the use of centrifugal pump. Feed water pumps usually run

intermittently and are controlled by a float switch or other similar level-sensing device

energizing the pump when it detects a lowered liquid level in the boiler substantially

increased. Some pumps contain a two stage switch. As liquid lowers to the trigger point

of the first stage, the pump is activated.

If the liquid continues to drop (perhaps because the pump has failed, its supply has

been cut-off or exhausted, or its discharge is blocked),the second stage will be

triggered. This stage may switch off the boiler equipment (preventing the boiler from

running dry and overheating), trigger an alarm or both.

6. Control valves-

Control Valves are the valves used within industrial plants and elsewhere to control

operating conditions such as temperature, pressure, flow and liquid level by fully or

partially opening or closing in response to signals received from controllers that

compares a “set point” to a “process variable” whose value is provided by sensors that

monitor changes in such conditions. The opening or closing of control valves is done by

means of electrical, hydraulic or pneumatic systems.

7. De-aerator-

A De-aerator is a boiler feed device for air removal and used to remove dissolved gases

from water to make it non-corrosive. A de-aerator typically includes a vertical domed

de-aeration section as the de-aeration feed water tank. A steam generating boiler

requires that the circulating steam, condensate and feed water should be devoid of

dissolved gases, particularly corrosive ones and dissolved or suspended solids. The

gases will give rise to corrosion of the metal (due to cavitations). The solids will deposit

on heating surfaces giving rise to localized heating and tube ruptures due to

overheating. De-aerator level and pressure must be controlled by adjusting control

valves-the level by regulating condensate flow and pressure by regulating steam flow.

Most de-aerators guarantee that if operated properly, oxygen in de-aerated water will

not exceed 7ppb by weight.

8. Feed Water Heater-

A feed water heater is a power plant component used to pre heat water delivered to a

steam generating boiler. Feed water heater improves the efficiency of the system. This

reduces plant operating costs and also helps to avoid thermal shock to boiler metal

when the feed water is introduced back into the steam cycle. Feed water heaters allow

the feed water to be brought upto the saturation temperature very gradually. This

minimizes the inevitable irreversibility associated with heat transfer to the working

fluid(water). A belt conveyer consists of two pulleys, with a continuous loop of

material- the conveyer belt that rotates around them. The pulleys are powered, moving

the belt and the material on the belt forward. Conveyer belts are extensively used to

transport industrial and agricultural material, such as grain, coal, ores, etc.

9. Pulverizer-

A pulverizer is a device for grinding coal for combustion in a furnace, in a coal based

fuel power plant.

10. Boiler Steam Drum-

Steam Drums are a regular feature of water tube boilers. It is reservoir of water/steam

at the top end of the water tubes in the water-tube boiler. They store the steam

generated in the water tubes and act as a phase separator for the steam/water mixture.

The difference in densities between hot and cold water helps in the accumulation of the

“hotter”-water/and saturated –steam into steam drum. Made from high-grade steel

(probably stainless) and its working involves temperatures 411’C and pressure well

above 350psi (2.4MPa). The separated steam is drawn out from the top section of the

drum. Saturated steam is drawn off the top of the drum. The steam will re-enter the

furnace in through a super heater, while the saturated water at the bottom of steam

drum flows down to the mud- drum /feed water drum by down comer tubes

accessories include a safety valve, water level indicator and fuse plug. A steam drum is

used in company of a mud-drum/feed water drum which is located at a lower level. So

that it acts as a sump for the sludge or sediments which have a higher tendency at the

bottom.

11. Super Heater-

A Super heater is a device in a steam engine that heats the steam generated by the

boiler again increasing its thermal energy and decreasing the likelihood that it will

condense inside the engine. Super heaters increase the efficiency of the steam engine,

and were widely adopted. Steam which has been superheated is logically known as

superheated steam; non-superheated steam is called saturated steam or wet steam;

Super heaters were applied to steam locomotives in quantity from the early 20th

century, to most steam vehicles, and so stationary steam engines including power

stations.

12. Economizers-

Economiser is mechanical devices intended to reduce energy consumption, or to

perform another useful function like preheating a fluid. The term economizer is used

for other purposes as well, e.g. air conditioning. Boiler heating in power plants. In

boilers, economizer are heat exchange devices that heat fluids , usually water, up to but

not normally beyond the boiling point of the fluid. Economizers are so named because

they can make use of the enthalpy and improving the boiler’s efficiency. They are a

device fitted to a boiler which saves energy by using the exhaust gases from the boiler

to preheat the cold water used for feed into the boiler (the feed water). Modern day

boilers, such as those in coal fired power stations, are still fitted with economizer which

is decedents of Green’s original design. In this context they are turbines before it is

pumped to the boilers. A common application of economizer is steam power plants is to

capture the waste hit from boiler stack gases (flue gas) and transfer thus it to the boiler

feed water thus lowering the needed energy input , in turn reducing the firing rates to

accomplish the rated boiler output . Economizer lowers stack temperatures which may

cause condensation of combustion gases (which are acidic in nature) and may cause

serious equipment corrosion damage if care is not taken in their design and material

selection.

13. Air Preheater-

Air preheater is a general term to describe any device designed to heat air before

another process (for example, combustion in a boiler). The purpose of the air preheater

is to recover the heat from the boiler flue gas which increases the thermal efficiency of

the boiler by reducing the useful heat lost by the flue gases. As a consequence, the flue

gases are also sent to the flue gas stack (or chimney) at a lower temperature allowing

simplified design of the ducting and the flue gas stack. It also allows control over the

temperature of gases leaving the stack (chimney).

14. Electrostatic Precipitator-

An Electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate device

that removes particles from a flowing gas (such As air) using the force of an induced

electrostatic charge. Electrostatic precipitators are highly efficient filtration devices,

and can easily remove fine particulate matter such as dust and smoke from the air

steam. ESP’s continue to be excellent devices for control of many industrial particulate

emissions, including smoke from electricity-generating utilities (coal and oil fired), salt

cake collection from black liquor boilers in pump mills, and catalyst collection from

fluidized bed catalytic crackers from several hundred thousand ACFM in the largest

coal-fired boiler application. The original parallel plate-Weighted wire design

(described above) has evolved as more efficient ( and robust) discharge electrode

designs were developed, today focusing on rigid discharge electrodes to which many

sharpened spikes are attached , maximizing corona production. Transformer –rectifier

systems apply voltages of 50-100 Kilovolts at relatively high current densities. Modern

controls minimize sparking and prevent arcing, avoiding damage to the components.

Automatic rapping systems and hopper evacuation systems remove the collected

particulate matter while on line allowing ESP’s to stay in operation for years at a time.

15. Fuel gas stack-

A Fuel gas stack is a type of chimney, a vertical pipe, channel or similar structure

through which combustion product gases called fuel gases are exhausted to the outside

air. Fuel gases are produced when coal, oil, natural gas, wood or any other large

combustion device. Fuel gas is usually composed of carbon dioxide (CO2) and water

vapor as well as nitrogen and excess oxygen remaining from the intake combustion air.

It also contains a small percentage of pollutants such as particulates matter, carbon

mono oxide, nitrogen oxides and sulfur oxides. The flue gas stacks are often quite tall,

up to 400 meters (1300 feet) or more, so as to disperse the exhaust pollutants over a

greater aria and thereby reduce the concentration of the pollutants to the levels

required by governmental environmental policies and regulations.

BMD

Boiler Maintenance

Department

BOILER

MAKE : Doosan Heavy Industries & Construction Co LTD, Korea.

DESIGNATION : Once-Thru in super Super-critical and Two-pass, balanced draft,

Outdoor, Radiant Reheat, Top support in sub-critical.

FURNACE SPECIFICATIONS

VOLUME : 21,462 m³

TYPE OF BOTTOM : Coutant

WIDTH : 18,816 mm

DEPTH : 18,144 mm

WATER WALL

Spiral Wall Tubes

Material : SA213T22

NO : 440

OD : 38.0 mm

Spacing : 50 mm

Vertical Wall

Material : SA213T22

NO : 1,320

OD : 34.0 mm

Spacing : 56 mm

SUPERHEATER

TYPE :Multi-stage with panel, Platen and pendant sections

REHEATER

TYPE :Multi-stage

ECONOMIZER

TYPE :Bare tube, Inline, Counter flow; 4 banks;

Assemblies :163

Tube O.D :50.8 mm

Type Material

Economizer SA210

Evaporator

Tube

Spiral SA213T22

Vertical SA213T22

Super heater

Tube

Primary SA213T23,

Secondary SA213T12, T23,

Final SA213T23, T91

Re-heater

Tube

Primary SA210C, T12,

Final SA213T23, T91

SUPER304H

Separator SA302C

Header SH SA335P9

RH SA335P9

A boiler is the central or an important component of the thermal power plant which

focuses on producing superheated steams that is used for running of the turbines which

in turn is used for the generation of electricity. A boiler is a closed vessel in which the

heat produced by the combustion of fuel is transferred to water for its conversation

into steam of the desired temperature & pressure.

The heat-generating unit includes a furnace in which the fuel is burned. With the

advantage of water-cooled furnace walls, super heaters, air heaters and economizers,

the term steam generator was evolved as a better description of the apparatus.

Boilers may be classified on the basis of any of the following characteristics:

Use Pressure Materials Size Tube Content Tube Shape and position Firing Heat Source Fuel Fluid Circulations Furnace position Furnace type General shape Trade name Special features.

Use: The characteristics of the boiler vary according to the nature of service performed.

Customarily boiler is called either stationary or mobile. Large units used primarily for

electric power generation are known as control station steam generator or utility

plants.

Pressure: To provide safety control over construction features, all boilers must be

constructed in accordance with the Boiler codes, which differentiates boiler as per their

characteristics.

Materials: Selection of construction materials is controlled by boiler code material

specifications. Power boilers are usually constructed of special steels.

Size: Rating code for boiler standardize the size and ratings of boilers based on heating

surfaces. The same is verified by performance tests.

Tube Contents: In addition to ordinary shell type of boiler, there are two general steel

boiler classifications, the fire tube and water tube boilers. Fire tube boiler is boilers

with straight tubes that are surrounded by water and through which the products of

combustion pass. Water tube boilers are those, in which the tubes themselves contain

steam or water, the heat being applied to the outside surface.

Firing: The boiler may be a fired or unfired pressure vessel. In fired boilers, the heat

applied is a product of fuel combustion. A non-fired boiler has a heat source other than

combustion.

Heat Source: The heat may be derived from (1) the combustion of fuel (2) the hot

gasses of other chemical reactions (3) the utilization of nuclear energy.

Fuel: Boilers are often designated with respect to the fuel burned.

Fluid: The general concept of a boiler is that of a vessel to generate steam. A few

utilities plants have installed mercury boilers.

Circulation: The majority of boilers operate with natural circulation. Some utilize

positive circulation in which the operative fluid may be forced 'once through' or

controlled with partial circulation.

Furnace Position: The boiler is an external combustion device in which the combustion

takes place outside the region of boiling water. The relative location of the furnace to

the boiler is indicated by the description of the furnace as being internally or externally

fired.

The furnace is internally fired if the furnace region is completely surrounded by water

cooled surfaces. The furnace is externally fired if the furnace is auxiliary to the boiler.

Furnace type: The boiler may be described in terms of the furnace type.

General Shape: During the evaluation of the boiler as a heat producer, many new

shapes and designs have appeared and these are widely recognized in the trade.

Trade Name: Many manufacturers coin their own name for each boiler and these names

come into common usage as being descriptive of the boiler.

Special features: some times the type of boiler like differential firing and Tangential

firing are described.

Categorization of Boilers:

Boilers are generally categorized as follows:

• Steel boilers

• Fire Tube type

• Water tube type

• Horizontal Straight tube

The boiler is generally used for power production are two types:-

1. Corner boiler

2. Front fire boiler

The boiler mainly has natural circulation of gases, steam and other things. They contain

vertical membrane water. The pulverized fuel which is being used in the furnace is

fixed tangentially. They consume approximately 700 ton.\hr of coal of about

1370kg\cm2 of pressure having temperature of 540оC.

The first pass of the boiler has a combustion chamber enclosed with water walls of

fusion welded construction on all four sides. In addition there are four water platens to

increase the radiant heating surface.

Beside this platen super heater reheater sections are also suspended in the furnace

combustion chamber. The first pass is a high heat zone since the fuel is burn in this

pass.

The second pass is surrounded by steam cooled walls on all four sides as well as roof of

the boiler. A horizontal super heater, an economizer & two air heaters are located in the

second pass.

The main components of a boiler and their functions are given below:

a) DRUM: It is a type of storage tank much higher placed than the level at which

the boiler is placed, and it is also a place where water and steam are separated. First

the drum is filled with water coming from the economizer, from where it is brought

down with the help of down-comers, entering the bottom ring headers. From there

they enter the riser, which are nothing but tubes that carries the water (which now is a

liquid-vapor mixture), back to the drum. Now, the steam is sent to the super heaters

while the saturated liquid water is again circulated through the down-comers and then

subsequently through the risers till all the water in the drum turns into steam and

passes to the next stage of heating that is superheating.

NOTE: for a 660 MW plant, the boiler does not employ any drum; instead the

water and steam go directly into the super heater because the pressure employed being

higher than the critical pressure of water on further stages of heating will eventually

turn completely into steam without absorbing any latent heat of vaporization since the

boiling part in the T-s curve no longer passes through the saturation dome rather its

goes above the dome.

Fig 1 : typical non regenerative

rankine cycle followed by sub -

critical power plants.

Fig 2 : typical non regenerative

rankine cycle followed by

super critical power plants.

b) SUPER HEATERS: The steam from the boiler drum is then sent for

superheating. This takes place in three stages. In the first stage, the steam is sent to a

simple super heater, known as the low temperature super heaters (LTSH), after which

the second stage consists of several divisional panels super heaters (DPSH). The final

stage involves further heating in the Platen super heaters (PLSH), after which the

steam is sent through the Main Steam (MS) piping for driving the turbine.

Superheating is done to increase the dryness fraction of the exiting steam. This is

because if the dryness fraction is low, as is the case with saturated steam, the presence

of moisture can cause corrosion of the blades of the turbine. Super heated steam also

has several merits such as increased working capacity, ability to increase the plant

efficiency, lesser erosion and so on. It is also of interest to know that while the super

heater increases the temperature of the steam, it does not change the pressure. There

are different stages of super heaters besides the sidewalls and extended sidewalls. The

first stage consists of LTSH (low temperature super heater), which is conventional

mixed type with upper & lower banks above the economizer assembly in rear pass.

The other is Divisional Panel Super heater which is hanging above in the first pass of

the boiler above the furnace. The third stage is the Platen Super heater from where the

steam goes into the HP turbine through the main steam line. The outlet temperature &

pressure of the steam coming out from the super heater is 540 degrees Celsius & 157

kg/cm2. After the HP turbine part is crossed the steam is taken out through an outlet as

CRH(Cold Re-heat steam) to be re-heated again as HRH(Hot Re-heat steam) and then

is fed to the IPT(Intermediate pressure turbine) which goes directly to the LPT(Low

pressure turbine) through the IP-LP cross-over.

c) WATER WALLS: The water from the bottom ring header is then transferred to

the water walls, where the first step in the formation of steam occurs by absorbing heat

from the hot interior of the boiler where the coal is burned continuously. This

saturated water steam mixture then enters the boiler drum.

In a 500 MW unit, the water walls are of vertical type, and have rifled tubing

whereas in a 660 MW unit, the water walls are of spiral type till an intermediate ring

header from where it again goes up as vertical type water walls. The advantage of the

spiral wall tubes ensures an even distribution of heat, and avoids higher thermal

stresses in the water walls by reducing the fluid temperature differences in the adjacent

tubes and thus minimizes the sagging produced in the tubes.

d) ECONOMIZER: The economizer is a tube-shaped structure which contains

water from the boiler feed pump. This water is heated up by the hot flue gases which

pass through the economizer layout, which then enters the drum. The economizer is

usually placed below the second pass of the boiler, below the Low Temperature Super

heater. As the flue gases are being constantly produced due to the combustion of coal,

the water in the economizer is being continuously being heated up, resulting in the

formation of steam to a partial extent. Economizer tubes are supported in such a way

that sagging, deflection & expansion will not occur at any condition of operation.

Figure depicting the difference between the vertical water wall

and the spiral water wall type of tubing where the vertical water

walls have the rifle type of tubes to increase the surface area

unlike the spiral ones that have plain, smooth surfaces.

e) DEAERATOR:

A deaerator is a device that is widely used for the removal of air and other

dissolved gases from the feedwater to steam-generating boilers. In particular,

dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam

systems by attaching to the walls of metal piping and other metallic equipment and

forming oxides (rust). Water also combines with any dissolved carbon dioxide to

form carbonic acid that causes further corrosion. Most deaerators are designed to

remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less.

There are two basic types of deaerators, the tray-type and the spray-type:

The tray-type (also called the cascade-type) includes a vertical domed

deaeration section mounted on top of a horizontal cylindrical vessel which serves

as the deaerated boiler feedwater storage tank.

The spray-type consists only of a horizontal (or vertical) cylindrical vessel

which serves as both the deaeration section and the boiler feedwater storage tank.

In addition to these there are several other smaller components attached to a boiler,

including several safety valves, which have their own special significance.

So briefly, the boiler functions this way. The water enters the boiler through the

economizer. From there it passes to the drum. Once the water enters the drum it comes

down the down comers to the lower inlet water wall headers. From the headers the

water rises through the water walls and is eventually turned into steam due to the

continuous heat being generated by the burners. As the steam is formed it enters the

steam drum. Here the steam and water is separated. The separators and dryers remove

the droplets of water and the cycle through the water walls is repeated. This cycle is

known as natural circulation cycle. In the forced circulation of water pumps are used

to maintain the flow of water.

the overall figure of the boiler depicting the flow of the fuel and the

gases along the given direction of the arrows .

ASSOCIATED SYSTEMS IN A POWER PLANT

There are several systems in a power plant which assist the main units to carry out

their functions properly:

1) PA FANS: The primary air fans are used to carry the pulverized coal particles

from the mills to the boiler. They are also used to maintain the coal-air temperature.

The specifications of the PA fan used at the plant under investigation are: axial flow,

double stage, reaction fan.

The PA fan circuit consists of:

a) Primary air path through cold air duct

b) Air pre-heater

c) Hot air duct

d) Mills

The model no. of the PA fan used at NTPC Sipat is AP2 20/12, where A refers to the

fact that it is an axial flow fan, P refers to the fan being progressive, 2 refers to the

fan involving two stages, and the numbers 20 and 12 refer to the distances in

decimeters from the centre of the shaft to the tip of the impeller and the base of the

impeller, respectively. A PA fan uses 0.72% of plant load for a 500 MW plant.

2) FD FANS: The forced draft fans, also known as the secondary air fans are used

to provide the secondary air required for combustion, and to maintain the wind box

differential pressure. Specifications of the FD fans are: axial flow, single stage,

impulse fan.

The FD fan circuit consists of:

a) Secondary air path through cold air duct

b) Air pre-heater

c) Hot air duct

d) Wind box

The model no. of the FD fan used at NTPC Sipat is AP1 26/16, where the

nomenclature has been described above. FD fans use 0.36% of plant load for a 500

MW plant.

3) ID FAN: An induced fan circuit consists of

a) Flus gas through water walls

b) Superwater

c) Re-heater

d) Platen super heater

e) low temperature super heater

f) Air pre-heater

g) Electrostatic precipitator

The main purpose of an ID fan is to suck the flue gas through all the above

mentioned equipments and to maintain the furnace pressure. ID fans use 1.41% of

plant load for a 500 MW plant.

4) AIR PRE-HEATERS: Air pre-heaters are used to take heat from the flue gases

and transfer it to the incoming air. They are of two types:

a) Regenerative

b) Recuperative

The APH used at NTPC Sipat is a Ljungstrom regenerative type APH. A

regenerative type air pre-heater absorbs waste heat from flue gas and transfers this

heat to the incoming cold air by means of continuously rotating heat transfer

elements of specially formed metal sheets. A bi-sector APH preheats the combustion

air. Thousands of these high efficiency elements are spaced and compactly arranged

within sector shaped compartments of a radially divided cylindrical shell called the

rotor. The housing surrounding the rotor is provided with duct connections at both

ends, and is adequately sealed by radial and axial sealing members forming an air

passage through one half of the APH and a gas passage through the other.

As the rotor slowly revolves the elements alternately pass through the air and gas

passages; heat is absorbed by the element surfaces passing through the hot gas

stream, then as the same surfaces pass through the air stream, they release the heat to

increase the temperature of the combustion of process air.

A single APH is divided into 4 parts: 2 PAPHs and 2 SAPHs. The P and S refer to

primary and secondary respectively. Each part is divided into two slots, one slot

carrying the primary/secondary air, and the other slot carrying the hot flue gases

coming from the 2nd

pass of the boiler. The PAPH is connected to the mills, whereas

the SAPH is connected to a wind box.

5) ELECTROSTATIC PRECIPITATORS: They are used to separate the ash

particles from the flue gases. In this the flue gas is allowed into the ESP, where there

are several metallic plates placed at a certain distance from each other. When these

gases enter, a very high

potential difference is

applied, which causes the

gas particles to ionize and

stick to the plates, whereas

the ash particles fall down

and are collected in a

hopper attached to the

bottom of the ESP. The

flue gas is allowed to cool

down and is then released

to the ID fan to be sent to

the chimney.

6) MILL: As the name suggests the coal particles are grinded into finer

sized granules. The coal which is stored in the bunker is sent into the mill,

through the conveyor belt which primarily controls the amount of coal required

to be sent to the furnace. It on reaching a rotating bowl in the bottom

encounters three grinding rolls which grinds it into fine powder form of

approx. 200 meshes per square inch. the fine coal powder along with the

heated air from the FD and PA fan is carried into the burner as pulverized coal

while the trash particles are rejected through a reject system.

Types of coal pulverizers:

Impact

Attrition

Crushing

Sometimes these pulverizers employ all the three techniques all together.

7) SEAL AIR FAN: The seal air fan is used near the mill to prevent the loss of

any heat from the coal which is in a pulverized state and to protect the bearings from

coal particle deposition.

8) WIND BOX: these acts as distributing media for supplying secondary/excess

air to the furnace for combustion. These are generally located on the left and and

right sides of the furnace while facing the chimney.

9) IGNITER FAN: Igniter fans which are 2 per boiler are used to supply air for

cooling Igniters & combustion of igniter air fuel mixture.

10) CHIMNEY: These are tall RCC structures with single & multiple flues. Here,

for I & II we have 1 chimney, for unit III there is 1 chimney & for units IV & V

there is 1 chimney. So number of chimneys is 5 and the height of each is 275 metres.

11) COAL HANDLING PLANT: This part of the thermal power plant handles all

the requirements of coal that needs to be supplied to the plant for the continuous

generation of electricity. Coal is generally transported from coal mines ( mostly

located in peninsular regions of India ) to Thermal power plant with the help of rail

wagons. A Single rail wagon can handle upto 80 tons of coal( gross weight) . When

these rail wagons reach the thermal plant the coal is unloaded with the help of wagon

tipplers. A wagon tippler is actually a huge J shaped Link pinned at its top. Powerful

motors are used to pull the ropes attached to an end which lets the wagon to rotate at

an angle of 135 degree. The coal falls down due to action of gravity into the coal

bunkers. Vibration motors then are used to induce the movement the coal through its

way. as the coal reaches the hopper section of the bunker , it is taken away by

conveyer belts to either the storage yard or to the assembly points where the coal

gets distributed on different conveyers. Initially, the size of coal is taken as 250mm

in size. The macro coal has to be converted into micro ( 25mm ) size coal for the

actual combustion. This is attained by using high pressure crushers located at the

coal handling plants. Here various metal are separated by various mechanisms. There

are various paths through which a coal can go to boiler section. These paths are

alternative such as A and B and only one is used at a time letting the other standby.

The conveyor belts are monitored with various mechanisms such as:

Emergency stop

swaying

Slipping

metal detectors

Coal on the conveyer belts moves into raw coal bunkers known as RC bunkers.

There are six raw coal bunkers for each unit of the plant. The coal from six RC

bunkers falls onto six RC feeders and moves to six Ball mills.

12) COAL BUNKER: These are in process storage used for storing crushed coal

from the coal handling system. Generally, these are made up of welded steel plates.

Normally, these are located on top of mills to aid in gravity feeding of coal. There

are 10 such bunkers corresponding to each mill.

13) ASH HANDLING PLANT: The ash produced in boiler is transported to ash

dump area by means of sluice type hydraulic ash handling system, which consists of:

Bottom Ash System: In the Bottom Ash system the ash slag discharged

from the furnace bottom is collected in two water impounded scraper troughs

installed below bottom ash hoppers. The ash is continuously, transported by

means of the scraper chain conveyor, on to the respective clinker grinders

which reduce the lump sizes to the required fineness.

Fly Ash System: In this system, Fly ash gets collected in these hoppers drop

continuously to flushing apparatus where fly ash gets mixed with flushing

water and the resulting slurry drops into the ash sluice channel. Low pressure

water is applied through the nozzle directing tangentially to the section of

pipe to create turbulence and proper mixing of ash with water.

Ash Water System: High pressure water required for B.A hopper quenching

nozzles, B.A hopper`s window spraying, clinker grinder sealing scraper bars,

cleaning nozzles B.A hopper seal through flushing, Economizer Hoppers`

flushing nozzles and sluicing trench jetting nozzles is tapped from the high

pressure water ring main provided in the plant area.

Ash Slurry System: Bottom Ash and Fly Ash slurry of the system is sluiced

up to ash slurry pump along the channel with the aid oh high pressure water

jets located at suitable intervals along the channel. Slurry pump section line

consisting of reducing elbow with drain valve, reducer and butterfly valve

and portion of slurry pump delivery line consisting of butterfly valve, Pipe

and fitting has also been provided.

14) REHEATER: The function of reheater is to reheat the steam coming out from

the high pressure turbine to a temperature of 540 degrees Celsius. It is composed of

two sections: the rear pendant section is located above the furnace arc & the front

pendant section is located between the rear water hanger tubes & the Platen

superheater section.

15) BURNERS: There are total 20 pulverised coal burners for the boiler present

here, & 10 of the burners provided in each side at every elevation named as

A,B,C,D,E,F,G,H,J,K. There are oil burners present in every elevation to fire the fuel

oil (LDO & HFO) during lightup.

Apart from these units and systems, piping is another important system which is

essential for the proper transfer of fluids of different types from one unit to another.

In a power plant, pipes are used to transfer steam, water, oil, air etc. from one unit to

the other. Some criteria for the selection of the pipes are given below:

1) The piping should be of necessary size to carry the required flow of fluids.

2) Pipes should be able to withstand the high temperatures and expansions

due to changes in temperatures.

3) Piping system should withstand the high pressures to which it may be

subjected.

For smooth and safe operation of the power plant it is desirable to use minimum

length of pipes, and they should be as direct and straight as possible.

Ways to increase the thermal efficiency of power plants:

The basic idea behind all the modifications to increase the thermal

efficiency of a power cycle is the same: Increase the average temperature at

which heat is transferred to the working fluid in the boiler, or decrease the

average temperature at which heat is rejected from the working fluid in the

condenser. That is, the average fluid temperature should be as high as possible

during heat addition and as low as possible during heat rejection.

Lowering the Condenser Pressure (Lowers Tlow,avg): Steam exists as a

saturated mixture in the condenser at the saturation temperature corresponding to the

pressure inside the condenser. Therefore, lowering the operating pressure of the

condenser automatically lowers the temperature of the steam, and thus the temperature

at which heat is rejected. The effect of lowering the condenser pressure on the Rankine

cycle efficiency is illustrated on a T-s diagram in Fig.1. For comparison purposes, the

turbine inlet state is maintained the same. The colored area on this diagram represents

the increase in net work output as a result of lowering the condenser pressure from P4

to P4’. The heat input requirements also increase (represented by the area under curve

2_-2), but this increase is very small. Thus the overall effect of lowering the condenser

pressure is an increase in the thermal efficiency of the cycle.

Fig:1 Effect of lowering of the condenser pressure on efficiency

Superheating the Steam to High Temperatures (Increases Thigh,avg):

The average temperature at which heat is transferred to steam can be increased without

increasing the boiler pressure by superheating the steam to high temperatures. The

effect of superheating on the performance of vapor power cycles is illustrated on a T-s

diagram in Fig.2. The colored area on this diagram represents the increase in the net

work. The total area under the process curve 3-3_ represents the increase in the heat

input. Thus both the net work and heat input increase as a result of superheating the

steam to a higher temperature. The overall effect is an increase in thermal efficiency,

however, since the average temperature at which heat is added increases.

Increasing the Boiler Pressure (Increases Thigh,avg): Another way of

increasing the average temperature during the heat-addition process is to increase the

operating pressure of the boiler, which automatically raises the temperature at which

boiling takes place. This, in turn, raises the average temperature at which heat is

transferred to the steam and thus raises the thermal efficiency of the cycle. The effect

of increasing the boiler pressure on the performance of vapor power cycles is

illustrated on a T-s diagram in Fig.3. Notice that for a fixed turbine inlet temperature,

the cycle shifts to the left and the moisture content of steam at the turbine exit

increases. This undesirable side effect can be corrected, however, by reheating the

steam, as discussed in the next section.

Fig:2 Effect of superheating the steam to high temperatures

LOSSES DURING OPERATION &

MAINTAINANCE OF PLANT

1) SURFACE ROUGHNESS: It increases friction & resistance. It can be due to Chemical deposits, Solid

particle damage, Corrosion Pitting & Water erosion. As a thumb rule, surface

roughness of about 0.05 mm can lead to a decrease in efficiency of 4%.

2) LEAKAGE LOSS:

Interstage Leakage

Turbine end Gland Leakages

About 2 - 7.5 kW is lost per stage if clearances are increased by 0.025 mm

depending upon LP or HP stage.

3) WETNESS LOSS:

Drag Loss: Due to difference in the velocities of the steam & water particles,

water particles lag behind & can even take different trajectory leading to losses.

Sudden condensation can create shock disturbances & hence losses.

About 1% wetness leads to 1% loss in stage efficiency.

Fig:3 Effect of increasing boiler pressure to increase efficiency

4) OFF DESIGN LOSSES:

Losses resulting due to turbine not operating with design terminal conditions.

Change in Main Steam pressure & temperature.

Change in HRH pressure & temperature.

Condenser Back Pressure

Convergent-Divergent nozzles are more prone to Off Design losses then

Convergent nozzles as shock formation is not there in convergent nozzles.

5) PARTIAL ADMISSION LOSSES:

In Impulse turbines, the controlling stage is fed with means of nozzle boxes, the

control valves of which open or close sequentially.

At some partial load some nozzle boxes can be partially open / Completely

closed.

Shock formation takes place as rotor blades at some time are full of steam & at

some other moment, devoid of steam leading to considerable losses.

6) LOSS DUE TO EROSION OF LP LAST STAGE BLADES:

Erosion of the last stage blades leads to considerable loss of energy. Also, It is

the least efficient stage.

Erosion in the 10% length of the blade leads to decrease in 0.1% of efficiency.

CONCLUSION

All the minor & major sections in the thermal project had been visited

& also understood to the best of my knowledge. I believe that this

training has made me well versed with the various processes in the

power plant. As far as I think there is a long way to go till we use our

newest of ever improving technologies to increase the efficiency

because the stocks of coal are dwindling and they are not going to last

forever. Its imperative that we start shouldering the burden together

to see a shining and sustainable future INDIA.