Impulse Turbine

Description of impulse turbine:

Wheel or Rotor :
The wheel or rotor is fitted over a shaft from which the useful power is available. It is a rotating element of the turbine on which moving blades are fixed.

Nozzle:
The nozzle is a passage for the flow of steam where pressure energy is converted into kinetic energy. Its main function is to produce a jet of steam with a high velocity.

Blades :
De Laval turbine shown in the image below is an example of simple impulse turbine.
In this, only one set of impulse type blades is rigidly fixed to the rim of the rotor or wheel. It converts the kinetic energy of steam into mechanical work.

Casing :
The casing is the outside cover of the steam turbine fixed over a frame. It is fitted with nozzle.

Working Principle of Impulse turbine :

If a jet of steam is discharged from a fixed nozzle at a high speed over a flat stationary plate, a steady force will be exerted over this plate. This force is nothing but an impulse. No work is done as the plate is fixed. But, if a number of such plates are fixed on the rim of a wheel, the wheel may be rotated due to the impulse of steam. Curved plates are used instead of flat plates to utilize greater amount of energy.

In the impulse turbine, steam is expanded in the fixed nozzle only. In the nozzle the velocity of steam increases with decrease of pressure. As the steam passes over the blades, the pressure remains constant with a decrease of velocity.

As the high velocity steam impinges against the baldes, it changes the momentum of jet causing impulsive force on the blades. The wheel is thus made to rotate in a definite direction.
Here the kinetic energy is converted into mechanical work, only by one set of blades. It is simplest type of impulse turbine.

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Steam Turbine | Prime Movers

The steam turbine is universally used as prime mover in steam power plants.

Flow over Blades:

The steam turbine obtains its motive power from the change of momentum of a jet of steam flowing over a curved blade. The steam jet, in moving over the curved surface of the blade, exerts a pressure on the blade owing to its centrifugal force. This centrifugal force is exerted normal to the blade surface as shown in figure and acts along the whole length of the blade.

The resultant of these centrifugal forces plus the effect of change of velocity is the motive force on the blade. It should be realized that the blade obtains no motive form any impact of the jet, because the blade is so designed that the steam jet will glide on and off the blade without any tendency to strike it. In principle, it is analogous to a train passing around a railway curve. The train exerts a radially outward force on the line due to the centrifugal force.

Moving and Fixed Blades

In a steam turbine, a number of small blades are fixed to the ring of a revolving wheel or rotor. Jets of steam of a high velocity are obtained by expansion through nozzles and are directed on to the blades. The effective force of these jets, acting on the blades, rotates the wheel.

In modern turbines several of the wheel of moving blades are keyed to the same shaft, having a ring of fixed blades between each wheel of moving blades. These fixed blades are fixed to the turbine casing. Their object is to receive the steam jet from the moving blade ring and to divert it on to the next ring of moving blades by changing its direction as shown. This diversion may continue over several rings of moving and fixed blades until the whole of the kinetic energy of the steam jet is expended.

In the next post, I will explain about the impulse turbine and reaction turbine.

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