- It is a rotatory mechanical device that extracts energy from a fluid flow and converts it into useful work.
- A turbine is a turbomachine that consists of at least one moving part known as rotor assembly. Rotor assembly has a shaft with blades mounted on it and moving fluid passes through the blades resulting in their movement and finally transfers rotational energy to the rotor.
- There is a casing enclosing blades in all turbines (Gas, steam, and water turbines) which controls the working fluid.
Steam Turbine: A steam turbine is a prime mover which converts high-pressure energy and high-temperature steam supplied by the steam generator into shaft work with the low-temperature steam exhausted to a condenser.
Classification of Steam Turbines
Based on working principle
- Impulse Turbines
- Reaction Turbine
Difference Between Impulse & Reaction Turbine
1. Pressure drop take place only in the nozzle and there is no drop of pressure in moving blade.
1. Pressure drop takes place both in fixed as well as moving blade.
2. The cross-section b/w the blade passage is input constant.
2. The blade cross-section passage is a mode converging type both for fixed and moving blade.
3. Blade design and manufacturing is simple so the impulse stages are of low cost.
3. Blode manufacturing and design is difficult so the reaction stages are difficult.
4. Power developed and efficiency is less.
4. Power developed and efficiency is more.
5. Partial admission of steam through the nozzle.
5. All round admission of steam through fixed blade.
6. Preferred mainly for small power need with low investment
6. In reaction preferred for large power need with heavy investment.
Based on shaft orientation
- Tandem Compound (TC):In this type of assembly, all the turbine are connected to a single rotor attached to a single regenerator.
- Cross Compounded (CC) :In this type of assembly, two separate shaft or rotors are used to run separate generator normally of different speed.
- Impulse turbines referred to as the single-rotor or multirotor are simple stages of turbines.
- In this type of turbine, the impulse blades are mounted on the shaft. Impulse blades can be characterized by half moon shape.
Single-Stage Impulse Turbine :
The single-stage impulse turbines are also known as the de Laval turbines after their inventor.
Compounding in Impulse Turbine:
- If there a high velocity of steam has flowed through one row of moving blades, it can produce a rotor speed of a maximum of 30000 rpm (approx) which is considered too high for practical use.
- It can be made of practical use by using more than one set of nozzles & rotors in a series attached to the shaft to absorb steam pressure or the jet velocity in number of stages instead of single stage. This method of control is called the compounding of turbines.
- These are two types of compounding:
(1). Velocity compounding or Curtis Staging:
Single-stage velocity compounded turbine has one set of nozzles and two rows of moving blades separated by the fixed blades.
(2). Pressure compounding or Rateau Staging:
- A reaction turbine consists of alternate rows of fixed blades (Nozzles) and rows of moving blades.
- The moving blades due to the impulse of steam received (caused by the change in momentum) and also because of expansion and acceleration of the steam relative to them.
- Pressure and enthalpy drop occur both in the stator and in the Rotor.
- A special design has half degree of reaction or 50% reaction is called as Parson's Turbine. It has symmetrical stator and rotor blades.
- The velocity triangles are symmetrical:
α1 = β2, β1 = α2,
V1 = Vr2, Vr1 = V2
η is greater in reaction turbine. The stage-wise Energy input is less, so there is the availability of more stages.
Steam Turbine Governing and Control
The main of governing is to maintain the turbine speed fairly constant irrespective of load. The main methods of steam turbine governing are as follows :
(a). Throttle governing
(b). Nozzle governing
(c). By-pass governing
(d). Either (a) combined (b) or (a) with (c).
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