Ntpc (national thermal power corporation) sipat mechanical vocational training report 2 haxxo24 i~i
Vocational Training Project Report
“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
Boiler Maintenance Department. IIT Kharagpur.
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.
INDIAN INSTITUTE OF TECHNOLOGY,
KHARAGPUR, WEST BENGAL.
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.
Mr. U. R. Verma,
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
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:
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
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
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
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.
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.
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
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
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
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.|~|
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.
VOLUME : 21,462 m³
TYPE OF BOTTOM : Coutant
WIDTH : 18,816 mm
DEPTH : 18,144 mm
Spiral Wall Tubes
Material : SA213T22
NO : 440
OD : 38.0 mm
Spacing : 50 mm
Material : SA213T22
NO : 1,320
OD : 34.0 mm
Spacing : 56 mm
TYPE :Multi-stage with panel, Platen and pendant sections
TYPE :Bare tube, Inline, Counter flow; 4 banks;
Tube O.D :50.8 mm
Secondary SA213T12, T23,
Final SA213T23, T91
Primary SA210C, T12,
Final SA213T23, T91
Header SH 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:
Tube Shape and position
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
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
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
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
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
1370kgcm2 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
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
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
. 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.
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
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,
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
3) ID FAN: An induced fan circuit consists of
a) Flus gas through water walls
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:
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
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:
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:
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
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
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
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%.
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
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.
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.|~|