What is Steam Turbine and how does it work ?
A steam turbine is a prime mover which converts heat energy into mechanical energy. In a conventional steam turbines cycle, water is used as the working fluid. The water is heated in a boiler by burning fuel. It evaporates into steam which is expanded in a turbine where mechanical power is generated. The steam generated is of high temperature and high pressure. The temperature is oftern in the 450 to 540 degrees centigrade range. The pressure ranges between 60 to 120 bar.
The essential parts of all steam turbines are nozzles and blades.
Both temperature and pressure fall as the steam passes through the turbine. The greater the pressure drop, the more energy can be captured from the steam. The more efficient power plants condense the steam back to water at the end of the turbine.
The theoretical maximum efficiency of a steam turbine based power plant is determined by the difference between the temperature at which the steam enters the high pressure turbine and the temperature at which it exits the low pressure turbine. The greater the temperature difference, the more energy can be extracted.
Steam turbines are finding greater use in process industries (like steel and chemicals) producing large quantities of waste heat. The waste heat produced can be used to generate steam as well as power. The capital cost of such plants can be slightly higher but the generation of power represents a useful by-product when the waste must be burnt in any case.
Steam turbines can also be deployed advantageously in industries with greater requirements of both steam and power. They are used in cogeneration or combined heat and power application where process steam is also used in the turbine to generate electricity. This also results in substantial improvements in overall process efficiency.
Parts of Steam Turbine:
3. Steam Chest
7. Emergency Stop Valve
8. Control Valves
9. Governing System
The blades serve for converting the thermal energy of the steam into mechanical energy. The turbine rotor, as carrier of blades, transmits the mechanical energy impulses on the running blades, transmits the mechanical energy impulses on the running blades in the form of rotational energy to the driven machinery.
The turbine rotor is constructed of chromium molybdenum alloy steel. The moving blades are mounted in the grooves made on the rotor. The turbine utilizes moving and non-moving blades. Non-moving blades are either attached directly to the turbine casing or else they are located in the carriers. Short strips of metal, shrouding, are attached to the outer edges of the non moving blades. This shrouding is used to assist in maintaining rigidity of the blades. An impulse wheel at the admission end of the rotor is equipped a different type of blading and serves as the regulation stage.
A tapped radial hole drilled into the stud of the front-bearing journal is intended for housing the over speed monitor. Cams protruding from the cylindrical surface of the stud at each side of the threaded hole serves as a protection against excessive axial displacement of the rotor. Whenever such an inadmissible axial shift of the rotor occurs during the operation of the turbine, either of the two cams will engage with the lever of the emergency tripping device and thus induce shutting off the steam supply.
A toothed wheel mounted by shrinking to the rear end of the rotor permits in connection with a mechanical barring gear slow turning of the rotor by hand in order to prevent warping.
The turbine casing is made of cast steel and is split horizontally, the joint being level with the rotor axis. The turbine casing houses and supports the turbine rotor, labyrinth seals, and bearings. The casing is cast in two halves and bolted together with a metal to metal fit. The casing of back pressure turbine is supported on separate bearing pedestals with the support surface level with the rotor axis. This ensures the position of the casing relative to the rotor always remains constant at all operating temperatures. The radial blade clearance thus being unchanged. In order to permit unrestricted horizontal expansion of the casing without moving it out center.
3. STEAM CHEST:
The steam chest houses the throttle valve assembly. Here main steam first enters the turbine. The throttle valve assembly regulates the amount of steam entering the turbine. After passing through the throttle valve, steam enters the nozzle block.
For supporting the weight of the turbine and to manage radial as well as axial alignment, there are two different types of bearings are being used.
A. JOURNAL BEARING:
Journal bearing maintain the radial alignment of the turbine and supports the weight of the rotor. Bearings are spherically seated allowing for slight radial misalignment during installation only. They are located on the forward and rear end of the turbine rotor.
B. THRUST BEARING:
Thrust bearing is located in the front end bearing pedestal, and is meant to take residual axial thrust present in the turbine which has not been eliminated by balance piston as well as to maintain the axial position of the rotor in the casing.
The thrust bearings are double acting, segment shoe, kingsbury type, having the advantages of compactness and uniform pressure distribution on all the thrust pads.
Glands with labyrinth seals are fitted at both ends where the shaft passes through the turbine casing. The labyrinths consists of sealing strips in the stationary part of the gland and grooves machined in the shaft.
6. EMERGENCY STOP VALVE:
The emergency stop valve is provided at the steam inlet to turbine. It is directly mounted on the casing with a view to reduce the quantity of steam entrapped between the stop valve and the control valves. In the event of sudden load throw off the more quantity of entrapped steam may tend to over speed the turbine.
7. CONTROL VALVES:
The control valves regulate the amount of steam flowing to the turbine according to the load. The cones of control valves are suspended from a beam. The beam is supported by two spindles which are raised and lowered through a system of levers by a servomotor arranged adjacent to the valves. The hanging distance of each control valve is adjusted with reference to its valve seat on the beam, so that when the beam is lifted, the valves open in a sequence and the steam is admitted progressively to various nozzle groups.
8. GOVERNING SYSTEM:
The most important and vital part of the steam turbine is governing system. The governing system should be simple in design, stable during operation and highly reliable. The governing system consists of a number of basic governing elements and protection. The speed governing elements are the speed sensor governor with the proper transformer amplifier, servomotor and governing valves.
Ways to improve efficiency:
More efforts are being made to improve the efficiency of steam turbines. The areas are
1. Super critical technology advances aiming for 50 percent efficiency.
2. Renovating and Upgrading for more value for money.
3. Combined Heat and Power for low cost, more flexibility.
4. Steam turbines in combined cycle, a new market.
5. Clean coal technologies FBC, PFBC, IGCC etc to improve the overall efficiency and to reduce the pollution level.
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M. K. Singh
Rajiv Saha Talukdar
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