Compounding of steam turbines

Compounding of steam turbines is the strategies in which energy from the steam is extracted in a number of stages rather than a single stage in a turbine. A compounded steam turbine has multiple stages i.e. it has more than one set of nozzles and rotors, in series, keyed to the shaft or fixed to the casing, so that either the steam pressure or the jet velocity is absorbed by the turbine in number of stages.

Contents

  • 1 Necessity
  • 2 Types of steam turbines
  • 3 Types of compounding
  • 4 Velocity compounding of Impulse Turbine
    • 4.1 Velocity Diagram
      • 4.1.1 Optimum Velocity
    • 4.2 Disadvantages of Velocity Compounding
  • 5 Pressure compounding of Impulse Turbine
    • 5.1 Velocity Diagram
    • 5.2 Disadvantages of Pressure Compounding
  • 6 Pressure-Velocity compounded Impulse Turbine
  • 7 Pressure compounding of Reaction Turbine
    • 7.1 Velocity Diagram
  • 8 See also
  • 9 References
  • 10 Further reading

Necessity

Compounding of steam turbine is used to reduce the rotor speed. It is the process by which rotor speed come to its desired value. A multiple system of rotors are connected in series keyed to a common shaft and the steam pressure or velocity is absorbed in stages as it flows over the blades.The steam produced in the boiler has sufficiently high enthalpy when superheated. In all turbines the blade velocity is directly proportional to the velocity of the steam passing over the blade. Now, if the entire energy of the steam is extracted in one stage, i.e. if the steam is expanded from the boiler pressure to the condenser pressure in a single stage, then its velocity will be very high. Hence the velocity of the rotor (to which the blades are keyed) can reach to about 30,000 rpm, which is too high for practical uses because of very high vibration. Moreover, at such high speeds the centrifugal forces are immense, which can damage the structure. Hence, compounding is needed. The high velocity steam just strikes on a single ring of rotor that causes wastage of steam ranging 10% to 12%. To overcome the wastage of steam compounding of steam turbine is used.

Types of steam turbines

  1. Impulse: There is no change in the pressure of the steam as it passes through the moving blades. There is change only in the velocity of the steam flow.
  2. Reaction: There is change in both pressure and velocity as the steam flows through the moving blades.

Types of compounding

In an Impulse steam turbine compounding can be achieved in the following three ways: -

  1. Velocity compounding
  2. Pressure compounding
  3. Pressure-Velocity Compounding

In a reaction turbine compounding can be achieved only by pressure compounding.

Velocity compounding of Impulse Turbine

Fig-1:Schematic Diagram of Curtis Stage Impulse Turbine

The velocity compounded Impulse turbine was first proposed by C G Curtis to solve the problem of single stage Impulse turbine for use of high pressure and temperature steam.

The rings of moving blades are separated by rings of fixed blades. The moving blades are keyed to the turbine shaft and the fixed blades are fixed to the casing. The high pressure steam coming from the boiler is expanded in the nozzle first. The Nozzle converts the pressure energy of the steam into kinetic energy. The total enthalpy drop and hence the pressure drop occurs in the nozzle. Hence, the pressure thereafter remains constant.

This high velocity steam is directed on to the first set (ring) of moving blades. As the steam flows over the blades, due to the shape of the blades, it imparts some of its momentum to the blades and loses some velocity. Only a part of the high kinetic energy is absorbed by these blades. The remainder is exhausted on to the ne... ...read more

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