Power Plant Engineering 4 U
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- emPOWERing Knowledge
--- emPOWERing Knowledge
Wednesday 24 July 2013
Tuesday 23 July 2013
Monday 22 July 2013
COMPONENTS OF STEAM TURBINE
1.
Casing or Cylinder:
It may be Barrel type
casing or Split type casing
a) Single shell casing
b) Multiple shell casing
2.
Rotor:
a) Built up rotor
b) Integral rotor
3.
Blades:
a) Impulse type blade
b) Reaction type blade
These blades
has three main parts:
a)
Aerofoil
b) Root
c) Shroud
4. Steam
chest:
It is
the portion in between turbine valves & rotor through which steam admission
to the turbine takes place.
5.
Turbine valves:
These
valves were mounted on the outer casing through which the steam is admitted to
the steam chest. These are of two types:
a) Stop valve
b) Control valve
6.
Bearing:
On which
the rotor supported during rotation.
7.
Labyrinths or Glands:
These
are the seals provided to arrest the leakage losses.
8.
Atmospheric relief valve or diaphragm:
This is
a rupturing diaphragm made of lead sheet is provided on the LP turbine outer
casing. This is allowed to rupture to avoid the over pressure at LP turbine
exhaust. Also some times called LP diaphragm.
9. Turning
gear device:
This is
a device attached to the rotor, which allows the rotor to rotate at constant
slow speed after stopping or before starting the machine. This is also called
barring gear.
10.
Governor or controller:
This is
a main device plays a important role during operation of the machine.
11.
Couplings:
This
connects the different shaft of the different turbines, so that power can be
added up to rotate the alternator.
SOMETHING ABOUT TURBINE
EVOLUTION OF STEAM TURBINE:
•
Concept of steam turbine was developed in 120BC.
•
Boiler-Turbine concept was developed in 1629.
•
Practical steam turbine was invented in 1889.
WORKING PRINCIPLE:
When steam is allowed to expand
through a narrow orifice, its heat energy (enthalpy) is converted to the
kinetic energy. This kinetic energy is converted to rotational energy through
the impact or reaction of the steam on the blades.
As the steam moves over the blades
its direction changes continuously & centrifugal pressure exerted on the
blades. This motive force is combination of centrifugal force & change of
moment and its direction is always normal to the blade surface.
TURBINE TYPES:
1. Impulse Turbine
2. Reaction Turbine
COMPOUNDING:
·
Velocity of steam is directly proportional to the square root of the
heat drop in the nozzle or fixed blade.
·
So complete expansion in one stage steam velocity touches the supersonic
speed.
·
To avoid this concept of compounding is adopted.
- As the pressure drops the
specific volume of the steam increases. To accommodate this huge volume of
steam the blade height gradually increases from one stage to the next
stage.
- As there is a pressure drop in
each stage, so this force the steam to pass through the clearances of
fixed & moving blades. Thus the inter stage sealing is needed to
improve the turbine efficiency.
- The pressure drop across the
stages creates an axial thrust on the rotor towards low pressure side.
This thrust is counter balanced by providing a balance piston & thrust
bearing.
DESIGN CONSIDERATION OF TURBINE:
·
To over come these facts generally a combination of both the types are
considered in designing a turbine.
·
Generally in high pressure side impulse stages were kept as leakage loss
is less & subsequently the percentage of reaction increases towards low
pressure side. In low pressure turbine complete reaction stages were
adopted.
CLASSIFICATION OF TURBINE:
1. According to the direction of flow:
a)
Axial flow turbine
b)
Radial flow turbine
c)
Single flow
d)
Double flow
2. According to the no. of cylinders:
a)
Single cylinder
b)
Double cylinder
c)
Three cylinder
d)
Four cylinder
3. According to the method of
governing:
a)
Throttle governing
b)
Nozzle governing
c)
Bypass governing
4. According to the principle of
action:
a)
Impulse
b)
Reaction
5. According to the heat balance
arrangements:
a)
Condensing with regeneration type
b)
Condensing type
c)
Back pressure type
6. According to the steam condition:
a)
Low pressure turbine (1 to 4ksc)
b)
High pressure turbine (4 to 40ksc)
c)
Intermediate pressure turbine ( >40ksc)
7. According to the shaft arrangement:
a)
Tandem compound
b)
Cross compound
8. According to the extraction type:
a)
Automatic extraction type
b)
Non-automatic type
TURBINE LOSSES:
·
External losses & Internal losses
·
External losses are bearing frictional losses & are same for all
turbines.
·
Internal losses:
a) Frictional loss due to fluid
flow
b) Leakage loss due inter stage passing
c) Leaving loss due to speed of
steam at exit from blade
·
Friction loss is minimised by providing smooth curved & aerofoil
shaped blades.
·
Leakages losses are minimised by providing inter stage sealing
arrangements in the blade tips.
·
Leaving loss is proportional to the square of the velocity &
minimised by reducing the velocity. The reduction in velocity is achieved by
increasing the blade height, so the
annular space increases towards the last stages
An Overview of a THERMAL POWER PLANT
"At present 54.09% or 93918.38 MW (Data
Source CEA, as on 31/03/2011) of total electricity production in India is
from Coal Based Thermal Power Station. A coal based thermal power plant
converts the chemical energy of the coal into electrical energy. This is
achieved by raising the steam in the boilers, expanding it through the turbine
and coupling the turbines to the generators which converts mechanical energy
into electrical energy."
INTRODUCTION:
In a coal based power plant coal is transported from coal mines to the
power plant by railway in wagons or in a merry-go-round system. Coal is
unloaded from the wagons to a moving underground conveyor belt. This coal from
the mines is of no uniform size. So it is taken to the Crusher house and
crushed to a size of 20mm. From the crusher house the coal is either stored in
dead storage( generally 40 days coal supply) which serves as coal supply in
case of coal supply bottleneck or to the live storage(8 hours coal supply) in
the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is
supplied to the Coal Mills by a Raw Coal Feeder. The Coal Mills or pulverizer
pulverizes the coal to 200 mesh size. The powdered coal from the coal mills is
carried to the boiler in coal pipes by high pressure hot air. The pulverized
coal air mixture is burnt in the boiler in the combustion zone.
Generally in modern boilers tangential firing system is used i.e. the coal nozzles/ guns form tangent to a circle. The temperature in fire ball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low Temperature Superheater, Platen Superheater and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.
Generally in modern boilers tangential firing system is used i.e. the coal nozzles/ guns form tangent to a circle. The temperature in fire ball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low Temperature Superheater, Platen Superheater and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.
BASIC PRINCIPLE:
Coal based thermal power plant works on the principal
of Modified Rankine Cycle.
Components of Coal Fired Thermal Power Station:
·
Coal Preparation
i)Fuel preparation system: In coal-fired power stations, the raw feed coal from the
coal storage area is first crushed into small pieces and then conveyed to the coal
feed hoppers at the boilers. The coal is next pulverized into a very fine
powder, so that
coal will undergo complete combustion
during combustion process.
ii)Dryers: they are used in order to remove the excess moisture from coal
mainly wetted during transport. As the presence of moisture will result in fall
in efficiency due to incomplete combustion and also result in
CO emission.
iii)Magnetic separators: coal
which is brought may contain iron particles. These iron particles may result in
wear and tear. The iron particles may include bolts, nuts wire fish plates etc.
so these are unwanted and so are removed with the help of
magnetic separators.
The coal we finally get after these above process are transferred
to the storage site.
Purpose of fuel storage is two –
Purpose of fuel storage is two –
·
Fuel
storage is insurance from failure of normal operating supplies to arrive.
·
Storage
permits some choice of the date of purchase, allowing the purchaser to take
advantage of seasonal market conditions. Storage of coal is primarily a matter
of protection against the coal strikes, failure of the transportation system
& general coal shortages.
·
Boiler
and auxiliaries:
A Boiler or steam generator essentially is a container into which
water can be fed and steam can be taken out at desired pressure, temperature
and flow. This calls for application of heat on the container. For that the
boiler should have a facility to burn a fuel and release the heat. The
functions of a boiler thus can be stated as:-
1.
To
convert chemical energy of the fuel into heat energy
2.
To
transfer this heat energy to water for evaporation as well to steam for
superheating.
The basic components of Boiler are: -
1.
Furnace
and Burners
2.
Steam
and Superheating
a. Low temperature superheater
b. Platen superheater
c. Final superheater
·
Economiser
It is located below the LPSH in the boiler and above pre heater.
It is there to improve the efficiency of boiler by extracting heat from flue
gases to heat water and send it to boiler drum.
Advantages of Economiser include
1) Fuel economy: – used to save fuel and increase overall
efficiency of boiler plant.
2) Reducing size of boiler: – as the feed water is preheated in
the economiser and enter boiler tube at elevated temperature. The heat transfer
area required for evaporation reduced considerably.
·
Air
Preheater
The heat carried out with the flue gases coming out of economiser
are further utilized for preheating the air before supplying to the combustion
chamber. It is a necessary equipment for supply of hot air for drying the coal
in pulverized fuel systems to facilitate grinding and satisfactory combustion
of fuel in the furnace
·
Reheater
Power plant furnaces may have a reheater section containing tubes
heated by hot flue gases outside the tubes. Exhaust steam from the high
pressure turbine is rerouted to go inside the reheater tubes to pickup more
energy to go drive intermediate or lower pressure turbines.
·
Steam
turbines
Steam turbines have been used predominantly as prime mover in all
thermal power stations. The steam turbines are mainly divided into two groups:
-
1.
Impulse
turbine
2.
Impulse-reaction
turbine
The turbine generator consists of a series of
steam turbines interconnected to each other and a generator on a
common shaft. There is a high pressure turbine at one end, followed by an
intermediate pressure turbine, two low pressure turbines, and the
generator. The steam at high temperature (536 ‘c to 540 ‘c) and pressure
(140 to 170 kg/cm2) is expanded in the turbine.
·
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 functions of a condenser are:-
1) To provide lowest economic heat rejection temperature for
steam.
2) To convert exhaust steam to water for reserve thus saving on
feed water requirement.
3) To introduce make up water.
We normally use surface condenser although there is one direct
contact condenser as well. In direct contact type exhaust steam is mixed with
directly with D.M cooling water.
·
Boiler
feed pump
Boiler feed pump is a multi stage pump provided for pumping feed
water to economiser. BFP is the biggest auxiliary equipment after Boiler and
Turbine. It consumes about 4 to 5 % of total electricity generation.
·
Cooling
tower
The cooling tower is a semi-enclosed device for evaporative
cooling of water by contact with air. The hot water coming out from the
condenser is fed to the tower on the top and allowed to tickle in form of thin
sheets or drops. The air flows from bottom of the tower or perpendicular to the
direction of water flow and then exhausts to the atmosphere after effective
cooling.
The cooling towers are of four types: -
1. Natural Draft cooling tower
2. Forced Draft cooling tower
3. Induced Draft cooling tower
4. Balanced Draft cooling tower
·
Fan or
draught system
In a boiler it is essential to supply a controlled amount of air
to the furnace for effective combustion of fuel and to evacuate hot gases
formed in the furnace through the various heat transfer area of the boiler.
This can be done by using a chimney or mechanical device such as fans which
acts as pump.
i) Natural
draught
When the required flow of air and flue gas through a boiler can be
obtained by the stack (chimney) alone, the system is called natural draught.
When the gas within the stack is hot, its specific weight will be less than the
cool air outside; therefore the unit pressure at the base of stack resulting
from weight of the column of hot gas within the stack will be less than the
column of extreme cool air. The difference in the pressure will cause a flow of
gas through opening in base of stack. Also the chimney is form of nozzle, so
the pressure at top is very small and gases flow from high pressure to low
pressure at the top.
ii) Mechanized draught
There are 3 types of mechanized draught systems
1)
Forced draught system
2)
Induced draught system
3)
Balanced draught system
Forced draught: – In this system a fan called Forced draught fan is installed at
the inlet of the boiler. This fan forces the atmospheric air through the boiler
furnace and pushes out the hot gases from the furnace through superheater,
reheater, economiser and air heater to stacks.
Induced draught: – Here a fan called ID fan is provided at the outlet of boiler,
that is, just before the chimney. This fan sucks hot gases from the furnace
through the superheaters, economiser, reheater and discharges gas into the
chimney. This results in the furnace pressure lower than atmosphere and affects
the flow of air from outside to the furnace.
Balanced draught:-In this system both FD fan and ID fan are provided. The FD fan
is utilized to draw control quantity of air from atmosphere and force the same
into furnace. The ID fan sucks the product of combustion from furnace and
discharges into chimney. The point where draught is zero is called balancing
point.
Ash
handling system
The disposal of ash from a large capacity power station is of same
importance as ash is produced in large quantities. Ash handling is a major
problem.
i) Manual handling: While
barrows are used for this. The ash is collected directly through the ash outlet
door from the boiler into the container from manually.
ii) Mechanical handling: Mechanical equipment is used for ash disposal, mainly bucket
elevator, belt conveyer. Ash generated is 20% in the form of bottom ash and
next 80% through flue gases, so called Fly ash and collected in ESP.
iii) Electrostatic precipitator: From air preheater this flue gases (mixed with ash) goes to ESP.
The precipitator has plate banks (A-F) which are insulated from each other
between which the flue gases are made to pass. The dust particles are ionized
and attracted by charged electrodes. The electrodes are maintained at
60KV.Hammering is done to the plates so that fly ash comes down and collect at
the bottom. The fly ash is dry form is used in cement manufacture.
·
Generator
Generator or Alternator is the electrical end of a turbo-generator
set. It is generally known as the piece of equipment that converts the
mechanical energy of turbine into electricity. The generation of electricity is
based on the principle of electromagnetic induction.
Advantages of coal based thermal Power Plant
·
They can respond to rapidly
changing loads without difficulty
·
A portion of the steam
generated can be used as a process steam in different industries
·
Steam engines and turbines can
work under 25 % of overload continuously
·
Fuel used is cheaper
·
Cheaper in production cost in
comparison with that of diesel power stations
Disadvantages of coal based thermal Power Plant
·
Maintenance and operating costs
are high
·
Long time required for erection
and putting into action
·
A large quantity of water is
required
·
Great difficulty experienced in
coal handling
·
Presence of troubles due to
smoke and heat in the plant
·
Unavailability of good quality
coal
·
Maximum of heat
energy lost
·
Problem of ash removing
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