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what is steam turbine? how do they work? what are the advantages and disadvantages of steam turbines?
what is steam turbine? how do they work? what are the advantages and disadvantages of steam turbines?
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 Page 1 The Steam Turbine 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. Page 2 The Steam Turbine 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. Page 3 The Steam Turbine 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! Page 4 The Steam Turbine Page 5 The Steam Turbine 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. Page 6 The Steam Turbine 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. Page 7 The Steam Turbine 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. Page 8 The Steam Turbine 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. Page 9 The Steam Turbine 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 Page 10 The Steam Turbine 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. Page 11 The Steam Turbine 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 Page 12 The Steam Turbine 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. Page 13 The Steam Turbine 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. Page 14 The Steam Turbine 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. Page 15 The Steam Turbine 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. Page 16 The Steam Turbine 9. Velocit diagram of steam at entrence & exit of blade of steam turbine Page 17
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