206 POWER PLANT ENGINEERING
As the pressure falls, the specific volume increases and hence in practice, the height of blades is
increased in steps i.e. say upto 4 stages it remains constant, then it increases and remains constant for the
next two stages.
In this type of turbine, the steam velocities are comparatively moderate and its maximum value is
about equal to blade velocity. In general practice, to reduce the number of stages, the steam velocity is
arranged greater than the blade velocity. In this case the leaving loss is about 1 So 2 per cent of the total
initial available energy. This type of turbine is used mostly in all power plants where it is great success.
An example of this type of turbine is the Parsons-Reaction Turbine. The power plants 30 MW and above
are all impulse-reaction type.
6.9 Advantages of Steam Turbine over Steam Engine
The various advantages of steam turbine are as follows :
(i) It requires less space.
(ii) Absence of various links such as piston, piston rod, cross head etc. make the mechanism
simple. It is quiet and smooth in operation,
(iii) Its over-load capacity is large.
(iv) It can be designed for much greater capacities as compared to steam engine. Steam turbines
can be built in sizes ranging from a few horse power to over 200,000 horse power in single units.
(v) The internal lubrication is not required in steam turbine. This reduces to the cost of
lubrication.
(vi) In steam turbine the steam consumption does not increase with increase in years of service.
(vii) In steam turbine power is generated at uniform rate, therefore, flywheel is not needed.
(viii) It can be designed for much higher speed and greater range of speed.
(ix) The thermodynamic efficiency of steam turbine is higher.6.10 Steam Turbine Capacity
The capacities of small turbines and coupled generators vary from 500 to 7500 kW whereas large
turbo alternators have capacity varying from 10 to 90 mW. Very large size units have capacities up to
500 mW.
Generating units of 200 mW capacity are becoming quite common. The steam consumption by
steam turbines depends upon steam pressure, and temperature at the inlet, exhaust pressure number of
bleeding stages etc. The steam consumption of large steam turbines is about 3.5 to 5 kg per kWh.
Turbine kW = Generator kW / Generator efficiency
Generators of larger size should be used because of the following reasons:
(i) Higher efficiency.
(ii) Lower cost per unit capacity.
(iii) Lower space requirement per unit capacity. 3.45.1 Nominal rating.
It is the declared power capacity of turbine expected to be maximum load.