Impulse Turbine

IMPULSE TURBINE

Principle

Refer Fig. 1. Impulse is defined as the force exerted on an object when a jet of fluid (liquid or gas) strikes the object with a velocity.

In an Impulse Turbine, the high pressure high temperature steam from the boiler expands through a Fixed Nozzle.

The high velocity jet of steam leaves the nozzle and is made to impact upon the Blades. The blades are fitted on the periphery of a Rotor. The rotor is mounted on a shaft. The shaft rotates due to the impulsive force exerted by the steam jet on the blades.

Description of De Laval Single Stage Impulse Turbine [Figs. 2(i) and (ii) ]

1. 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.

2.  Nozzle: Nozzle (fixed) is a passage for flow of steam where pressure energy is converted into kinetic energy. Its function is to produce a jet of steam with a high velocity.

3. Blades: In this turbine, only one set of moving blades is rigidly fixed to the rim of the rotor of wheel. It converts the kinetic energy of steam into mechanical work.

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

Working

In the De Laval Single Stage 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 blades, 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 energy, by one set of moving blades.

Fig. 3 shows the pressure-velocity diagram for the above impulse turbine.

 

1. COMPOUNDING OF IMPULSE TURBINES

The disadvantage of De-Laval impulse turbine is its extremely high speed, of the order of 30,000 rpm. It cannot be employed for practical purposes. To reduce the high speed, more than one set of blades are used. This is called Compounding of Impulse Turbine.

In the compounding method, the steam jet velocity or the steam pressure is absorbed in stages as it flows over the rotor blades. When steam velocity is absorbed in stages, it is called Velocity Compounding Impulse Turbine. When steam pressure is absorbed in stages, it is known as Pressure Compounding Impulse Turbine.

 

2. VELOCITY COMPOUNDING IMPULSE TURBINE

See Fig. 4(i). In a Two Stage Velocity Compounding Impulsive Turbine (Curtis Turbine), moving and fixed blades are placed alternately. Moving blades are fitted with the wheel. Fixed blade is fitted with casing.

Steam is expanded in the nozzle from the boiler pressure to condenser pressure, to a high velocity. It is then passed over the first moving blade M₁ . Only a portion of the high velocity of steam jet is absorbed by this blade. The remainder is exhausted on to the next fixed or guide . This fixed blade changes the direction of the steam jet.

The steam jet is then passed on to the next moving blade M2. A further portion of steam velocity is absorbed by this second moving blade.

The process is then repeated as the steam flows over the remaining pairs of blades until practically all the velocity of the jet has been absorbed and the kinetic energy is converted into mechanical work.

Note that all the pressure drop takes place in the nozzle itself. And the pressure remains constant, as the steam flows over the blades. Hence, the turbine is an impulse turbine.

Fig. 4(ii) shows the pressure velocity diagram for the two stage velocity compounding impulse turbine.

 

3. PRESSURE COMPOUNDING IMPULSE TURBINE

Fig. 5 shows the Pressure Velocity Diagram of a 2 Stage Pressure Compounding Impulse Turbine.

In this type, the expansion of steam takes place in more than one set of nozzles. Each set of nozzles is followed by a set of moving blades.

The total pressure drop of the steam does not take place in the first set of nozzles. But, it is divided up equally between all the sets of nozzles.