Turbine is a device which transforms pressure and temperature energy of steam into mechanical energy. It contains stator & rotor blades. Stator is connected to casting & turbine and perform like a nozzle in which pressure and steam temp is converted into high velocity of steam and then this high velocity of steam when strikes the rotor produce rate of change of momentum i.e. is force on blade and this force produces torque on rotor and hence rotor rotates on its axis. Note that rotor is mounted on shaft hence shaft also rotated with same rpm of water which is our desire need.
Any turbines consist mainly four parts:
Casing is simply cover of whole turbine in which all changes take place. As we know vacuum is created inside turbine system when rotor is rotating then this casing helps to maintain it. Casing is that part of turbine which is stationary hence plays a very vital role; it holds the nozzles of turbines parts, stator blades and does not allow the steam to flow outside in atmosphere. Note that casing plays a very good role during compounding of turbine.
Rotor of turbine is made of uniform strength because it has to rotate continuously on its axis for a large period of time. If it is of unknown thickness then due to unequal radial hoop and longitudinal stresses which are dominant i.e. maximum at center and keep on decreasing towards circumference may cause damage of rotor. Rotor must not contain any pin hole at any point.
Nozzles used in turbine are only to increase the velocity of steam. It is simply a device which converts pressure and temperature energy of steam ino kinetic energy of steam into kinetic energy of steam. As we know during rate of change of momentum on blades as larger the velocity of incoming steam the developed rate of change of momentum is so large. Nozzles are mainly of two kinds:
- Convergent-Divergent Nozzle
- Divergent Nozzle
Blades are that part of turbine where the steam strikes and rate of change of momentum takes place. Blades are fitted on the circumference of rotor. Generally two kinds of blades are used:
- Symmetric blades-Impulse turbine
- Asymmetric blades-Reaction turbine
Types of Turbine
Turbines are generally classified into two main categories:
- Impulse turbine
- Reaction turbine
Impulse turbine are those kind of turbine in which expansion of steam takes place only in nozzles not on blades of rotor. When steam passes on rotor blades its pressure remain constant if functional losses are negligible. Blades of this kind of turbine are made of symmetrical i.e. both inlet and outlet ends are same.
e.g:- Curtes, Zolley
Reaction turbines are those kinds in which expansion of steam takes place partially in nozzles and on blades also i.e. pressure of steam reduces when steam passes on blades. Blades of this turbine are made asymmetrical i.e. both the ends are of different construction.
Compounding of Turbine
Compounding of turbine is done to make and control speed of turbine rotor for practical use. The high speed of rotor is controlled by changing its form to increase its torque so that heavy load can be rotated.
Compounding is classified as:
- Pressure compounding- In pressure compounding pressure of steam is expanded in different nozzle stags as a result velocity is developed in each stage.
- Velocity Compounding- In velocity compounding once the expansion takes place.
- Pressure and Velocity Compounding- It consists of both compounding.
Modern turbine contains 3 parts and stages. They are:
- High pressure turbine
- Intermediate pressure turbine
- Low pressure turbine
Steam from boiler main super heater goes through main stream line to high pressure turbine where transformation of energy takes place i.e. pressure and temperature of steam is converted into mechanical work of rotor and then the exhaust steam is converted into mechanical work of rotor and then the exhaust steam of H.P. turbine is again through C.R.H. line passed to reheater of boiler. Steam again gained temperature goes to intermediate pressure turbine through H.R.H. line. I.P. turbine performs same as H.P. turbine performs. A pipe of steam from I.P. turbine is given to low pressure turbine and finally the exhaust steam from L.P. turbine goes to condenser. As we know torque developed and power developed at rotor is directly depend on the pressure reduction in stage of turbine hence exhaust steam from L.P. turbine is taken at very low pressure.
The exhaust steam of L.P. turbine is passed to condensed where it is cooled by cooling water from D.M. tank and to create vacuum in condenser ejectors are used. Water from condenser through condensate pump passes to gland cooler, drain cooler and then to L.P. heaters where an extraction from L.P. turbine is used to heat the water and then it is passes to dearator tank.
Lubricating Oil System
Turbines rotate on its axis and hence have bearings. Journal bearing is used to lift the rotor and shaft on its proper position perpendicular to axis of shaft and rotor where as thrust bearing is used to hold the turbine so that it could not move parallel to axis of turbine. There is also present baring gear between L.P. turbine and generator.
Steam continuously strikes the rotor part and rotor part is also rotating hence cooling is necessary for rotor. Bearing also requires cooling. Lubricating oil performs following purposes:
- Lubrication of rotating part
- To�keep rotor cool
- To function the control valve by controlling steam
- To function the stop valve
Before starting and after tripping of turbine it is rotated by help of motor or br oil turbine known as baring gear. As we know rotor is so heavy so that axis of rotor is maintained a jacking oil pump is used to maintain its axis by giving pressure to shaft of turbine to lift up by oil. For lubrication of bearing three types of bearing are used:
- When turbine is on baring gear then from AC and DC oil pumps oil is taken and then by passing to oil cooler it is given to baring gear.�Pressure and temperature of this oil is 1kg/cm2 and 40C.
- A V solary of pump which gives oil to bearings and also help in control and safety.
- When turbine is on then to cool front part of rotor a main oil pump is present which provide lubrication as well as control and safety.
Turbine Gland Seal System
Seal is given so that steam in turbine could not (from M.S.I.) go to the bearing of turbine. Sealing in turbine is done by gland seal system of system. Simply seal prevents the leakage steam from turbine to come outside in atmosphere. Sealing also prevents the entering of air in L.P. turbine because there is vacuum inside it and air goes from high pressure to low pressure side. Gland seal steam is taken from auxiliaries steam system & after sealing this steam is absorbed into gland cooler.
- In a 210 MW unit on load of main steam going to H.P. & I.P. is used to seal the L.P. turbine.
- In a 110 MW unit gland steam of H.P. & I.P. turbine go to chimney steam condenser whereas gland steam of turbine go to Gland steam condenser.
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