Question

Explain with the help of neat sketch a single-stage impulse turbine. Also explain the pressure and velocity variations along the axial direction

Answer 1

Compounding of steam turbines: The method in which energy from steam is extracted in more than single stage is called Compounding. A multi-stage turbine i.e having more than one set of rotors and nozzles is called compounded turbine.

1. Velocity Compounding of Impulse Turbine:The velocity compounded impulse turbine has moving and fixed blades. The moving blades are keyed to turbine shaft and fixed blades are fitted to casing. The high pressure steam from boiler is expander in nozzle where pressure energy is converted into kinetic energy.
The high velocity steam is directed on first set of moving blades and as steam flows over the blade it imparts some of its momentum to blades and loses some velocity. Some part of high K.E is absorbed by blades and there is no change in velocity of steam as it passes through fixed blades. The steam then goes to next set of moving blades and this process is repeated until all the energy of steam is absorbed. The figure below shows the velocity compounding of impulse turbine.



Here: Pi and Po are pressure of steam at inlet and outlet and 
           Vi and Vo are velocities of steam at inlet.

2. Pressure Compounding of Impulse Turbine(Rateau turbine):
This is used to solve the problem of high blade velocity in the single-stage impulse turbine.
It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing and the blades are keyed to the turbine shaft. In this type of compounding, steam is expanded more than once(as in velocity compounding).




Here the high pressure steam is fed to nozzle where it is partially expanded i.e pressure decreased velocity increased and when this steam is passed over the set of blades, where almost all its velocity is absorbed and pressure remains constant during this period and this process is repeated until condenser pressure is achieved.
The symbols in the figure have same meaning as above.