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STEAM TURBINE
1.INTRODUCTION
A steam turbine is a mechanical device that extracts thermalenergy from pressurized steam, and converts it into rotary motion. Its
modern manifestation was invented by Sir Charles Parsons in 1884.
Definitions ofsteam turbine:
y turbine in which steam strikes blades and makes them turny A steam turbine is a mechanical device that extracts thermal energy from
pressurized steam, and converts it into rotary motion. Its modern manifestationwas invented by Sir Charles Parsons in 1884.
y A system of angled and shaped blades arranged on a rotor through which steamis passed to generate rotational energy. Today, normally used in power stations
y A device for converting energy of high-pressure steam (produced in a boiler) intomechanical power which can then be used to generate electricity.
y Equipment unit flown through by steam, used to convert the energy of the steaminto rotational energy.
A machine for generating mechanical power in rotary motion from the
energy of steam at temperature and pressure above that of an available
sink. By far the most widely used and most powerful turbines are thosedriven by steam. Until the 1960s essentially all steam used in turbine cycles
was raised in boilers burning fossil fuels (coal, oil, and gas) or, in minorquantities, certain waste products. However, modern turbine technology
includes nuclear steam plants as well as production of steam supplies fromother sources.
The illustration shows a small, simple mechanical-drive turbine of a
few horsepower. It illustrates the essential parts for all steam turbines
regardless of rating or complexity: (1) a casing, or shell, usually divided atthe horizontal center line, with the halves bolted together for ease of assembly
and disassembly; it contains the stationary blade system; (2) a rotor
carrying the moving buckets (blades or vanes) either on wheels or drums,with bearing journals on the ends of the rotor; (3) a set of bearings attachedto the casing to support the shaft; (4) a governor and valve system forregulating the speed and power of the turbine by controlling the steam flow,
and an oil system for lubrication of the bearings and, on all but the smallest
machines, for operating the control valves by a relay system connected with
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the governor; (5) a coupling to connect with the driven machine; and (6)
pipe connections to the steam supply at the inlet and to an exhaust system at
the outlet of the casing or shell.
Steam turbines are ideal prime movers for driving machines requiring
rotational mechanical input power. They can deliver constant or variable
speed and are capable of close speed control. Drive applicationsinclude centrifugal pumps, compressors, ship propellers, and, mostimportant, electric generators.
Steam Turbines Basics
Though "Steam Turbines" might sound like a technical term, most of
the things we do everyday would be impossible to do without this wonderfultechnology in power generation. Nature does not have sockets from wherepower plants pull out electricity to run your laptop or charge your iPod!Energy needs to be converted to electricity or electrical energy, from its
natural occurrences. Steam Turbines are devices that help in the production
of electricity, by converting mechanical energy into useful electrical energy!The Steam Turbine was invented by Parson, more than a century ago, and it
has gone through numerous changes to become an effective power
generator in today's power plants.
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2.THE MODERN STEAM TURBINE
The steam turbine continues to be a major factor in electric power
generation throughout the world. Even nuclear plants use the heat from a
controlled nuclear chain reaction to produce needed steam. In the United States,
more than 88 percent of all electricity is produced by steam turbines
Steam is no remnant of the Industrial Revolution. Even nuclear power plants employ steam
technology.
As mentioned earlier, there are basically three stages of matter: Solid, liquid
and gas. Each stage is held together by a different level of molecular force.
With water, gaseous steam takes up space due to its molecules being furthest
apart. However, when enough pressure is applied to steam, an amazing thing
happens. The molecules are forced together to the point that the water becomes
more like a liquid again, while retaining the properties of a gas. It is at this point
that it becomes a supercritical fluid.
Many of today's power plants use supercritical steam, with pressure and
temperature at the critical point. This means supercritical steam power plants
operate at much higher temperatures and pressures than plants using subcritical
steam. Water is actually heated to such a high pressure that boiling does not even
occur.
The resulting high-pressure fluid of supercritical steam provides excellent
energy efficiency. With the aid of high pressure, supercritical steam turbines canbe driven to much higher speeds for the same amount of heat energy as
traditional steam power. They also release less CO2 exhaust into the atmosphere.
Additionally, new high-pressure boilers built with rocket technology are being
developed to further control the levels of CO2 emitted. Some boilers will even cool
the steam back into a liquid and channel it into the ground to capture emissions.
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3.Principle of Operation and Design
In reciprocating steam engine, the pressure of energy of steam is used to
overcome external resistance and dynamic action of the steam is negligibly small. Steam
engine may be return by using the full pressure without any expansion or drop ofpressure in the cylinder.
How Does A Steam Turbine Work?
A steam turbine, as we see from its name, uses steam to rotate its blades. The
rotary motion of the blades is used to rotate the armature of the generator, and the
movement of the armature in a magnetic field results in the production of a current
(electricity) in the armature! The steam turbine has come a long way from its initial
design: there is the single flow steam turbine, the multiple flow steam turbines, the
reaction steam turbine, the impulse-reaction steam turbine, and the impulse turbine. It
has been the object of research and interest of many engineers and scientists like De
Laval, Parson, and Curtis. Heat energy from a coal thermal power plant or a nuclear
power plant is used to boil waiter, and convert it into steam at high pressure. This high
pressure steam is directed to the turbine blade thus causing the blade to rotate!
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4.Steam Turbine Parts Know Your Turbine!
Steam turbines are machines that are used to generate mechanical (rotational
motion) power from the pressure energy of steam. Steam turbines are the most popular
power generating devices used in the power plant industry primarily because of the highavailability of water, moderate boiling point, cheap nature and mild reacting properties.
The most widely used and powerful turbines of today are those that run on steam. From
nuclear reactors to thermal power plants, the role of the steam turbine is both pivotal
and result determining.
What Goes Into The Construction Of Steam Turbines?
A steam turbine basically has a mechanical side, and an electrical side to it. The
mechanical components include the moving parts (mechanical), such as the rotor, the
moving blades, the fixed blades, and stop valves, while the electrical side consists of the
generator and other electrical components to actually convert the energy into a usable,easily transferable form.
Blades:
For starters, a simple turbine works just like a windmill. Only, in the steam turbines of today,
rather than striking the blades directly, the blades are designed in such a way as to produce
maximum rotational energy by directing the flow of the steam along its surface. So the primary
component that goes into a steam turbine is its blades. The blades of a steam turbine are designed
to behave like nozzles, thus effectively tapping both the impulse and reaction force of the steam forhigher efficiency. Nozzle design itself is a complex process, and the nozzle shaped blade of the
turbine is probably one of the most important parts in its construction. The blades are made at
specific angles in order to incorporate the net flow of steam over it in its favor. The blades may be
of stationary or fixed and rotary or moving or types.
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Shafts:
The shaft is a power transmitting device and is used to transmit the rotational
movement of the blades connected to it at one end via the rotor to the coupling, speed
reducer or gear at the other end.
Outer Casing:
The steam turbine is surrounded by housing or an outer casing which contains
the turbine and protects the device components from external influence and damage. It
may also support the bearings on which the shafts rest to provide rigidity to the shaft.
Usually split at the center horizontally, the casing parts are often bolted together for
easy opening, checking and steam turbine maintenance, and are extremely sturdy and
strong.
Governor:
The governor is a device used to regulate and control or govern the output of the
steam turbine. This is done by means of control valves which control the steam flow into
the turbine in the first place.
Oil System:
A steam turbine has thousands of moving parts and all these parts not only have
to move in high velocities, but also need to be protected from wear and tear over the
years. This is done by effective lubrication by the oil system, which governs the
pressure, flow and temperature of the turbine oil, the bearing oil and lubrication of
other moving parts.
Pipes:
The pipe is an all important steam turbine component that brings the steam from
the boiler to the turbine. This has to be done without an appreciable loss in pressure,
and at the same time, must be able to withstand all these pressures safely. The pipes
should be easy to clean and are prone to deposits on their inner surfaces. Deposits on
the inner surface of the steam pipe reduce the net steam flow area, throwing forth a
negative effect on the efficiency.
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5.How are Steam Turbines Classified?
The first steam turbine, at its time indeed did spark off the industrial revolution
through out the west. However, the turbine at that time was still an inefficient piece ofheavy weighing high maintenance machine. The power to weight ratio of the first
reciprocating steam turbine was extremely low, and this led to a great focus improving
the design, efficiency and usability of the basic steam turbine, the result of which are
the power horses that currently produce more than 80% of todays electricity at power
plants!
How are Steam Turbines Classified?
Steam Turbines can be classified on the basis of a number of factors. Some of the
important methods of steam turbine classification are enunciated below:
On the basis of Stage Design:Steam turbines use different stages to achieve their ultimate power conversion goal.
Depending on the stages used by a particular turbine, it is classified as Impulse Turbine,
or Reaction type.
On the Basis of the Arrangement of its Main Shaft:Depending on the shaft arrangement of the steam turbine, they may be classified
as Single housing (casing), tandem compound (two or more housings, with shafts that
are coupled in line with each other) and Cross compound turbines (the shafts here are
not in line).
On the Basis of Supply of Steam and Steam ExhaustCondition:
They may be classified as Condensing, Non Condensing, Controlled or Automatic
extraction type, Reheat (the steam is bypassed at an intermediate level, reheated and
sent again) and Mixed pressure steam turbines (they have more than one source of
steam at different pressures).
On the basis of Direction of Steam Flow:They may be axial, radial or tangential flow steam turbines.
On the Basis of Steam Supply:Superheated steam turbine or saturated steam turbine.
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6.Basic types of turbine
The two most basic and fundamental types of steam turbines are the impulse
turbine and the impulse reaction turbine.
6.1 The Impulse Turbine:The impulse turbine consists of a set of stationary blades followed by a set of
rotor blades which rotate to produce the rotary power. The high pressure steam flows
through the fixed blades, which are nothing but nozzles, and undergo a decrease in
pressure energy, which is converted to kinetic energy to give the steam high velocity
levels. This high velocity steam strikes the moving blades or rotor and causes them to
rotate. The fixed blades do not completely convert all the pressure energy of the steam
to kinetic energy, hence there is some residual pressure energy associated with the
steam on exit. Therefore the efficiency of this turbine is very limited as compared to the
next turbine we are going to review- the reaction turbine or impulse reaction turbine.
How Does An Impulse Turbine Work?
The impulse turbine was one of the basic steam turbines. It involved striking of
the blades by a stream or a jet of high pressure steam, which caused the blades of the
turbine to rotate. The direction of the jet was perpendicular to the axis of the blade. It
was realized that the impulse turbine was not very efficient and required high pressures,
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which is also quite difficult to maintain. The impulse turbine has nozzles that are fixed to
convert the steam to high pressure steam before letting it strike the blades.
Impulse turbine mechanism
Impulse turbine Mechanism deals with the Impulse force action-reaction.
As we all know the Newton 3rd law of motion," Every action has equal and
opposite reaction", the same is work on this.
As the water fall on the blade of the rotor it generate the impact force on the
blade surface, The blade tends to give the same reaction to the fluid, but the rotor is
attached to the rotating assembly, it absorb the force impact and give the reaction in
the direction of the fluid flow. Thus the whole turbine rotates.
The rotation speed of the turbine depends on the fluid velocity, more the fluid velocity,
greater the rotation speed, and greater the speed means more power generation.
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6.2 The Reaction Turbine
The reaction turbine is a turbine that makes use of both the impulse and the reaction of
the steam to produce the rotary effect on the rotors. The moving blades or the rotors here are
also nozzle shaped (They are aerodynamically designed for this) and hence there is a drop in
pressure while moving through the rotor as well. Therefore in this turbine the pressure drops
occur not only in the fixed blades, but a further pressure drop occurs in the rotor stage as well.
This is the reason why this turbine is more efficient as the exit pressure of the steam is lesser,
and the conversion is more. The velocity drop between the fixed blades and moving blades is
almost zero, and the main velocity drop occurs only in the rotor stage.
How REACTION TURBINE works?
Reaction Turbines
In the reaction turbine, the rotor blades themselves are arranged to form
convergent nozzle Reaction Turbines
In the reaction turbine, the rotor blades themselves are arranged to form
convergent nozzles. This type of turbine makes use of the reaction force produced as
the steam accelerates through the nozzles formed by the rotor. Steam is directed onto
the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire
circumference of the rotor. The steam then changes direction and increases its speed
relative to the speed of the blades. A pressure drop occurs across both the stator and
the rotor, with steam accelerating through the stator and decelerating through the
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rotor, with no net change in steam velocity across the stage but with a decrease in both
pressure and temperature, reflecting the work performed in the driving of the rotor.
This type of turbine makes use of the reaction force produced as the steam
accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by
the fixed vanes of the stator. It leaves the stator as a jet that fills the entire
circumference of the rotor. The steam then changes direction and increases its speed
relative to the speed of the blades. A pressure drop occurs across both the stator and
the rotor, with steam accelerating through the stator and decelerating through the
rotor, with no net change in steam velocity across the stage but with a decrease in both
pressure and temperature, reflecting the work performed in the driving of the rotor.
Difference betw
een impulse turbine & reactio
n turbine?In an impulse turbine, the water (or steam) hits the blades and continues almost
straight through as in a jet engine. In a reaction turbine, the water hits a semicircular
cup and is completely reversed in path, normally dropping down the center with little or
no momentum left. These are rarely used with gases because of having to get the
output out of the way, but they work especially well with water at lower pressure as
when the dam supplying the water is not very high. Both kinds are used in various
situations.
What are the advantages of impulse cum reaction turbine over pure
impulse and pure reaction turbine?
The difference between impulse and reaction turbine goes here......
1) In case of an impulse turbine the pressure remains same in the rotor or
runners, but in case of reaction turbine the pressure decreases in runners as well as
stators also.
2) In case of impulse turbine the pressure drop happens only in the nozzle part by
means of its kinetic energy. In case of Reaction one the stators those are fixed to the
diaphragm act as a nozzle.
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7.How Can A Steam Turbine Be Improved?
A steam turbine has thousands of miniature components. From the gigantic
blades that drive the rotor, to the bearings and nuts that keep the machine in place, thesteam turbine has tremendous scope for improvement and effective design of every
part plays a significant role in improving the turbines overall efficiency. Some of the
areas where a lot of research goes into are those such as nozzle design, aerodynamic
blade design, lubrication engineering, heat transfer mechanisms, part cooling,
fabrication and part machining, pipe flow mechanisms, metallurgy etc.
Design of steam turbine machine parts such as nozzles and blades to make them
aerodynamic using computational fluid dynamics has gained a lot of steam as a field in
itself! A small advancement in the blade design could help in increasing efficiency
tremendously. Blade design with Computational Fluid Dynamics or CFD focuses on
reducing the local profile oriented loss on a Quasi 3 Dimensional (Q3D) basis. The design
of proper inlet ducts from the turbines based on their operating time, economic
considerations, size of the network and size of the turbine is also equally important. In
this case, since the flow is highly unsteady and complex, the effects and degree of non
uniformity in the flow has to be controlled to a large extent or predicted and taken care
of suitably. Choosing proper materials for the different steam turbine components and
parts is also an important aspect of design. The use of different lightweight yet strong
and thermally resistant alloys to make steam turbine blades and moving parts is of very
high importance. This also brings about the issue that the material should be as freefrom erosion as possible and should not succumb to rust and other chemical changes
while under operation. Technologies such as anti erosion blade shields bear testimony
to this.
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8.Steam Turbine Applications
The Steam turbines of today are mostly used in the power production field. Steam
turbines are used to efficiently produce electricity from solar, coal and nuclear power plants
owing to the harmlessness of its working fluid, water/steam, and its wide availability.Modern steam turbines have come a long way in increasing efficiency in performance and
more and more efforts are being made to try and reach the ideal steam turbine conditions,
though this is physically impossible! Almost every power plant in the world, other than
hydro electric power plants, that use turbines that run on water (the Francis, Pelton
turbines also have the influence of steam turbines) , use steam turbines for power
conversion. With all the scientific advancement in power generation being attributed to
them, steam turbines really have changed the way the world moves!
Steam turbines are devices which convert the energy stored in steam into rotational
mechanical energy. These machines are widely used for the generation of electricity in a
number of different cycles, such as:
y Rankin cycley Reheat cycley Regenerative cycley Combined cycle
Utility Steam Turbine Applications
Applications for utility Steam Turbines are applied for control of straight condensing,
reheat and non-reheat steam turbines up to 300MW. These upgrades may include
integrated generator control for generator protection and excitation/ AVR upgrades,
utilizing the latest commonly available industry-standard digital equipment.
Industrial application of steam turbine
Applications of Industrial Steam Turbines cover all straight condensing, non-
condensing, and automatic extraction steam turbines. Specific design features are
incorporated to address control issues often unique to process plants including paper mills,
oil refineries, chemical plants, and other industrial applications, generator and mechanical
drive.
Some of the worlds largest turbines manufacturing companies that are seeing the
rewards of research and steam turbine advances are coming together to develop highly
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efficient turbines. The collaboration of Mitsubishi Heavy Machinery and General Electric
Energy (GE Energy) for the conceptualization and design of a highly efficient next-
generation steam turbine for its inception in combined cycle gas turbine power plants
recently has further proved that there is still a lot to be achieved in steam turbine related
research and development, and that the scope for improvement can be much higher.
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9.Velocit diagram of steam at entrence & exit of blade of
steam turbine
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