Difference between Impulse and Reaction Turbine

In case of an impulse turbine the pressure remains same in the rotor or runners, but in case of reaction turbine the pressure decreases in runners as well as stators also.

In case of impulse turbine the pressure drop happens only in the nozzle part by means of its kinetic energy. In case of Reaction one the stators those are fixed to the diaphragm act as a nozzle. 

Impulse Turbine	Reaction Turbine
At the inlet only kinetic energy is available. I.e. only momentum transfer.	At the inlet, both kinetic and pressure energy are available.
Steam expands in the fixed blade (which act as a nozzle) itself. The entire pressure drop occurs at nozzles. Therefore, pressure remains constant while it passes over the blades.	The steam expands continuously in both fixed and the moving blades as it passed above them. Thus, the pressure drop occurs in a gradual way during flow over both blades.
Water casing is not necessary.	Air-tight casing is necessary.
Pressure is same at both end of moving blade and is equal to atmospheric pressure.	Inlet pressure is higher than outlet pressure.
In impulse turbine, flow area of moving blade is constant, since there is no expansion of steam.	In reaction turbine, the moving blade do have change in flow area (convergent type).They are shaped like nozzle and accelerate the steam.
Blades are symmetrical about plane of turbine shaft. Therefore, the manufacturing of blade is simple	Blades are asymmetrical.
In impulse turbine, due to great pressure drop, the operating speed is very high.	In the reaction, small pressure drop due to turbine, the operating is low.
Due to high pressure drop on the nozzles, the number of stages are less. The size of an impulse for the power output turbine is comparatively small.	Due to the small pressure drop at each stage, the number of stages are more for the same pressure drop as in impulse turbine. Therefore the size of the reaction for the same output power turbine is large. Reaction turbines are only multi-stage turbines.
Operate at high water head.	Operate at low water head.
Less space per unit power	More space takes per unit power
Turbine Impulse is suitable for small powers generation. More hydraulic efficiency	Reaction turbine is suitable for medium and higher powers generation.
Examples: Pelton wheel, Banki turbine	Examples: Kaplan turbine, Francis turbine, Fourneyron turbine.

  In an impulse turbine, the water (or steam) hits the blades and continues almost straight through as in a jet engine. In a reaction turbine, the water hits a semicircular cup and is completely reversed in path, normally dropping down the center with little or no momentum left. These are rarely used with gases because of having to get the output out of the way, but they work especially well with water at lower pressure as when the dam supplying the water is not very high. Both kinds are used in various situations. 

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Velocit diagram of steam at entrence & exit of blade of steam turbine