Steam turbine
a turbine in which a high-velocity jet of steam rotates a bladed disc or drum.
Working and construction
when the steam is allowed to expand through a narrow orifice, it assumes kinetic energyat the expense of enthalpy (heat energy).this kinetic energy of steam is charged to mechanical (rotational) energy through the impact (impulse) a reaction of steam against the blades.It should be realized that the blade of the turbine obtains no moving force from the static pressure of the steam or from any impact of the steam jet.The blades are designed in such a way,that the steam will guide on and off the blade without and tendency to strike it.
as the steam moves over the blade,its direction is continuously changing and centrifugal pressure exerted as the result is normal to the blade surface at all points.the total motives force acting on the blades is thus the resultant of all the centrifugal forces plus change of momentum.this causes the rotational motion of blades.
CONSTRUCTIONAL DETAILS:
The constructional details of the turbine includes the HP,IP,and LP turbines.
H.P.TURBINES:
The outer casing of the HP turbine is of barrel type. This avoids mass accumulation due to absence of flanges. It consists
H.P outer casing
H.P inner casing
H.P rotor
The inner casing carries guide blades and is kinematically supported.The spaces between inner and outer casings are filled by sealing rings. The inner casing is fixed in the horizontal and vertical planes in the outer casings so that it can freely expand radial in all directions and axially from a fixed point. The barrel construction permits rapid starts-ups and higher rates of load changes due to absence of high thermal stress .Barrel type casings are also easy to cast which means the casings can be exceptionally good quality .The connection with the piping is madethrough breach nuts. This arrangement provides ease of opening the joint during maintainance.
I.P.TURBINE:
IP Turbine consists of
IP outer casing
IP Inner casing
IP rotor
The IP casing is split horizontally and is of double shell and double flow construction,with the inner casing carrying the guide and kinematically supported with in the outer casing. The construction provides flexibility for choosing the location of bleed steam point to suit the best thermal efficiency. The reheated steam enters the inner casing through top and bottom.
The arrangement confines the high steam temperature to the admission branch of the casing while the joint of the outer casing is only subjected to lower pressure and temperature at the exhaust of the inner casing.
The hydralic turning gear blades are located on the coupling of the IP Rotor.
LP TURBINE:
LP Turbine consists of
- LP outer casing
- LP Inner-outer casing
- LP Inner-Inner casing
- LP Rotor
The LP turbine is of triple shell fabricated construction .The outer casing consists of the front and rear end walls;two side members called longitudinal griders and top cover.The twin Shell inner casing is supported kinemayically at each end by two support arms resting on the side members of the outer casting.
The inner shell of the inner casting carries the guide blade carrier of the first stage of the turbine.Rings of guide carries,which constitutes the guide blade carries,which constitutes the remaining stages of turbine ,are bolted to the middle inner outer casing.
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