Steam Prime Movers (Turbine) in Thermal Power Plant
- Steam prime movers are either reciprocating steam engines or steam turbines. Due to reciprocating motion steam engines have become obsolete in now-a-days, and steam turbines are usually used as prime movers.
- They have several advantages over steam engines as prime mover. It has higher thermodynamic efficiency.
- Its construction is simple, there is no need of piston rod mechanism and slide valves, no fly wheel is required.
- They can be built in large sizes as much as 1000 MW. No wearing action is involved. Maintenance is much simple. Problems of vibration are also much less compared to steam engines.
- According to the action of steam on moving blades the steam turbines are of two types namely impulse and reaction type.
- They differ in working principle. In impulse turbines as shown in Figure (A) the steam expands in nozzles only and when passing over the blades, its pressure remains constant. The jet passes over several rings of moving blades until its kinetic energy is expanded.
- In reaction type turbines as shown in bap Figure (B) the steam does not expand in nozzles, but expand as it flows over the blades. The blades will therefore act also as nozzles.
Figure (A) 
Figure (B) - The expansion of steam as it flows over the blades is adiabatic. Any friction losses between the steam and blades are converted into heat, which in turn will reheat the steam.
- The effect of this, is to dry or super heat the steam as it flows over the blades.
- The final velocity of steam will be extremely high if it is expanded in a single nozzle from the boiler pressure down to the condenser pressure.
- If jet is now passed over a single blade ring and its kinetic energy is absorbed by this ring, it will be found that the speed of this blade ring is too high for practical purpose.
- But this problem is solved in steam turbines, as no reduction gear is required for the reaction type turbines because the speed being relatively low.
- The steam turbines used are of two types namely “Axial flow” and ‘Radial flow” type according to the type of flow of steam.
- In axial flow type, the steam flows over the blades in a direction parallel to the axis of the wheel.
- In radial flow turbines, the blades are arranged radially so that the steam enters at the blade tip nearest the axis of the wheel and flows towards the circumference.
- Commercial turbines use series combination of impulse and reaction types because steam can be used more efficiently by using impulse and reaction blading on the same shaft.
- Steam is expanded through the turbine from a high pressure at the throttle valve to a back pressure corresponding to a vacuum of 71 to 73.5 cm Hg or an absolute pressure of 5 to 2.5 cm Hg.
The steam turbines having horizontal shaft are of the following types
(i) Condensing or non-condensing.
(ii) Single shaft or multiple shaft.
(iii) Single cylinder or multiple cylinder.
(iv) Single flow or multiple flow.
(v) Extracting or non-extracting.
(vi) Controlled or uncontrolled extraction.
(vii) Reheat or non-reheat.
(viii) Throttle or multiple nozzle governed.
(ix) Constant or variable speed.
(x) Single or double shell.
The size of the unit installed depends upon several factors such as capital cost, spares required, load factor and peak load of the system.