The disadvantage of De Laval type of turbine is that it’s extremely high speed, of the order of 30,000 rpm, 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 compound impulse turbine”. When “steam pressure” is absorbed in stages it is known as “pressure compound impulse turbine”.

Velocity Compound Impulse Turbine:

In velocity compound impulse turbine, moving and fixed blades are placed alternately. Moving blades are fitted with the wheel while the fixed blades are fitted with the casing.

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

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

It should be noted that the entire pressure drop takes place in the nozzle itself, the pressure remaining constant, as the steam flows over the blades. Hence the turbine is an impulse turbine. The Curtis turbine is an example of velocity compound impulse turbine.

Pressure compound impulse turbine:

In this type the expansion of steam takes place in more than one set of nozzles and 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 is divided up equally between all the sets of nozzles.