STEAM TURBINES (DOC) by sanj2681

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what is steam turbine? how do they work? what are the advantages and disadvantages of steam turbines?

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									The Steam Turbine



                                STEAM TURBINE
                                   1. INTRODUCTION
          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.

     Definitions of steam turbine:

           turbine   in   which    steam    strikes   blades   and    makes     them    turn

           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.

           A system of angled and shaped blades arranged on a rotor through which steam
            is passed to generate rotational energy. Today, normally used in power stations

           A device for converting energy of high-pressure steam (produced in a boiler) into
            mechanical power which can then be used to generate electricity.

            Equipment unit flown through by steam, used to convert the energy of the steam
     into rotational energy.


           A machine for generating mechanical power in rotary motion from the
     energy of steam at temperature and pressure above t hat of an available
     sink. By far the most widely used and most powerful turbines are those
     driven 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 minor
     quantities, certain waste products. However, modern turbine technology
     includes nuclear steam plants as well as production of steam supplies from
     other 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 at
     the 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 attached
     to the casing to support the shaft; (4) a governor and valve system for
     regulating 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 small est
     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 applications
     include centrifugal pumps, compressors, ship propellers, and, most
     important, 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 wonderful
     technology in power generation. Nature does not have sockets from where
     power 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, liqu id
     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 can
     be 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 of
     pressure 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 high
     availability 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 for
     higher 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.

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     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 appreci able 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 of
     heavy 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 res ult of which are
     the power horses that currently produce more than 80% of today’s 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 Exhaust
          Condition:
            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.


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         On the Basis of Steam Supply:
           Superheated steam turbine or saturated steam turbine.

                             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
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     turbine to rotate. The direction of the jet was per pendicular to the axis of the blade. It
     was realized that the impulse turbine was not very efficient and required high pressures,
     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

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     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.
             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 between impulse turbine & reaction 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 pr essure 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, the
     steam turbine has tremendous scope for improvement and effective design of every
     part plays a significant role in improving the turbine’s 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 car e
     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 free
     from 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:

                Rankin cycle

                Reheat cycle

                Regenerative cycle

                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.

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             Some of the world’s largest turbines manufacturing companies that are seeing the
     rewards of research and steam turbine advances are coming together to develop highly
     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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