STEAM TURBINES (DIPLOMA LEVEL)
Steam turbine is a mechanical device, which is used for converting thermal energy of steam to mechanical energy.
Steam turbines are mainly classified into two types:
- Impulse turbine: The energy available at inlet of turbine is kinetic energy.
Ex: De-Laval turbine.
- Reaction turbine: The energy available at inlet of turbine is partially kinetic energy and partially pressure energy.
Ex: Parson’s reaction turbine.
De-Laval turbine (Simple impulse turbine): It consists of shaft, rotor, blades and converging – diverging nozzle.
The rotor is keyed to the shaft.
Around the circumference of rotor number of blades are connected with equal distance.
The geometrical shape of the blade is “cup” or profile.
Thickness of the blade is more at the center.
The blades are arranged such that the area of the flow remains constant while the steam is passing between the two blades.
In front of the blades about 3 to 4 nozzles are arranged with equal distance around the circumference of the rotor (as shown in Fig. 2)
The nozzles are arranged such that the steam enters to the blade at the angle ‘α’ with a velocity V1. Therefore it develops two components of velocities namely:
- Horizontal component of velocity (Vw1)
- Vertical component of velocity (Vf2)
Combined velocity triangles
The horizontal component of velocity is causing for driving the turbine, whereas vertical component of velocity is causing for carrying the steam towards the condenser.
CONDETION FOR MAXIMUM EFFICIENCY
From this relation it is very clear that, the blade efficiency completely depending on “ρ”.
Maximum work output per kg of steam
Force causing for carrying the steam towards the condenser is also called axial thrust (Fth), (It means the axial thrust is acting parallel to the turbine shaft axis.)
Stage efficiency :- The product of blade efficiency and nozzle efficiency is called stage efficiency.
Compounding of steam turbines (Impulse turbines)
The main aim of compounding of steam turbine is to reduce the speed of the turbine by expanding the steam in more than one stage.
Types of compoundings
- Velocity compounding (Curtis turbine)
- Pressure compounding (Rateau turbine)
- Pressure – velocity compounding
The blades which are connected to rotor are called movable blades (MB) and the blades which are connected to stator (It is not keyed to shaft) is called fixed blades (FB or SB).
The pressure and velocity are changing in the following manner, while the steam is passing between the blades and through the nozzle.
While the steam passing through the nozzle pressure decreases but velocity increases.
In the movable blade velocity decreases but pressure remains constant.
Both pressure and velocity remains constant in the fixed blade.
Velocity compounding (Curtis turbines)
Vi = Initial velocity
Vb = Back velocity
Pi = Initial pressure
Pb = Back pressure
PRESSURE COMPOUNDING (Rateau Turbine)
Pressure – Velocity compounding:
Reaction Turbine ( Parson’s Reaction Turbine)
The reaction turbines are constructed with two types of blades.
- Rotor blades
- Stator blades
The geometric shape of both blades is air foil (or) aerofoil.
Both stator and Rotor blades are arranged such that the area of flow is convergent (decreasing).
The pressure decreases continuously while the steam is expanding in the rotor blades and stator blades.
Because of drop of pressure in the stator blade section absolute velocity of the steam increases where as in rotor blade section relative velocity increases.
Pressure and velocity change in reaction turbines.
Differences between Impulse and Reaction turbines
|1. Works with only kinetic energy.||1. Works by Kinetic energy and pressure energy.|
|2. Blade shape is cup / profile.||2. Blade shape is aerofoil.|
|3. Pressure remains constants in both FB and MB.||3. Pressure continuously decreases.|
|4. Blades are less efficient.||4. Blades are more efficient.|
|5. Relative velocity at outlet is less than or equal to relative velocity at inlet.||5. Relative velocity at outlet is always more than inlet relative velocity.|
|6. Suitable for low power generation.||6. Suitable for high power generation.|
|7. Compounding is required.||7. Compounding is not required.|
REHEAT FACTOR (RF): The ratio between cumulative isentropic enthalpy drop and overall isentropic enthalpy drop is called Reheat factor.
Reheating is mainly due to internal friction between the blade surface and steam particles.
T – S diagram for 3-stage turbine
Bleeding (or) Regeneration:
The method of extracting a part of the steam from the turbine (before complete expansion) for the purpose of preheating the feed water is called bleeding.
From state 2 some quantity of steam is extracted (before complete expansion) for the purpose of heating the feed water to increase the feed water temperature. Therefore, the feed water is entering to the boiler at high temperature causing decreasing heat input to the boiler.
The main intension of regeneration is to increase the temperature of feed water (water supplied to the boiler).
Because of regeneration the thermal efficiency of steam power plant increases.
The method of heating the steam in between two stages of turbine expansion by sending the steam back to the boiler at constant pressure is called reheating.
HPT = High-Pressure Turbine
LPT = Low-Pressure Turbine
The main intension of reheating is to decrease the moisture content of steam in the last stages of the turbine.
Because of reheating the network output of thermal power plant increases.
T – S diagram for Reheating:
① – ② Isentropic expansion in HPT.
② – ③ Reheating at P=K.
③ – ④ Isentropic expansion in LPT.
④ – ⑤ Condensation at P=K.
⑤ – ⑥ Isentropic pumping.
⑥ – ① Constant pressure boiling process.
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