Comparison of Impulse and Reaction Turbines

Comparison of Impulse and Reaction Turbines:



Impulse Turbine
Reaction turbine
01. Energy available at the inlet is in the form of kinetic energy
01. Energy available at the inlet is in the form of kinetic and pressure energy
02. Pressure acting buckets is constant and atmospheric throughout the action of water on the runner till it is discharged to the tail race.
02. Water enters the runner with excess pressure and hence both pressure and velocity of water change as water passes through the runner.
03. Water does not fill the vanes and casings and air has free access to the buckets.
03. Water fills completely the vanes and buckets throughout the turbine operation.
04. Casing is used only to protect water from splashing and to guide water to the tail.
04. Casing is essential to send the unit from atmospheric condition as the turbine inlet pressure is much higher than the outlet pressure.
05. Turbine unit is installed above the tail race level.
05. Turbine unit is completely submerged in water bellow the tail race level.
06. Flow regulation does not result in possible loss.
06. Flow regulation results in possible loss.
07. As water passes over the guide vanes, its relative velocity reduces slightly or remains the same.
07. As water glides over the vanes, its relative velocity increases due to continuous drop in pressure during flow through the blade passages.









                     

Comparison of Impulse turbine and Impulse-reaction turbine:



Impulse turbine
Impulse reaction turbine
01. Steam expands completely in the nozzle.
01. Steam expands partially in the moving blades and partially in the fixed blades.
02. Pressure of steam remains constant during its flow through the blade passages.
02. Pressure of steam is not constant during its flow through moving and fixed blade passages.
03. Steam flow over the blades is uniform due to symmetrical blade profile.
03. Steam flow over the blades varies due to aerofoil section (unsymmetrical) blade profile.
04. Due to complete pressure drop in the nozzle, steam velocity and rotor speed are very high, resulting in centrifugal stresses acting on the blades. This may result in blade failure.
04. Since pressure drop occurs partially in the fixed blades, steam velocity and rotor speed are not high. Hence stresses developed are less.
05. The initial and final velocity of steam over the blades is constant or the final relative velocity can never be greater than the initial value, i.e. Vr₁ ≥ Vr₂
05. The initial and final relative velocities over the blades are not constant and the final relative velocity is always greater than the initial relative velocity.
06. As pressure drop in each stage is large, number of stages required is less. Hence turbine is compact.
06. Due to small pressure drop in each stage, it requires large number of stages. Hence size of turbine is bigger.
07. Part load efficiency of turbine is poor.
07. Part time efficiency of the turbine is good due to more efficient blades.
08. Speed of turbine is very high and hence requires compounding.
08. Speed of turbine is low and hence does not require compounding.
09. Turbine is compact and hence occupies less space.
09. Turbine is bigger and hence requires more space.
10. It is installed in small capacity power plants.
10. It is generally used in medium and large capacity power plants.


Inward flow turbine vs. Outward flow Reaction Turbine:



Inward Flow Reaction Turbine
Outward Flow Reaction turbine
01. Water flows the outer periphery of wheel towards the wheel centre and discharges at the inner periphery.
01. Water enters the wheel centre (inner periphery) and flows outward and discharges at the outer periphery.
02. The centrifugal head (U₂² - U₁²)/2 imparted to water as it flows through the turbine runner is negative, as U₂ is smaller than U₁
02. Water enters the runner with excess pressure and hence both pressure and velocity of water change as water passes through the runner.
03. Due to negative centrifugal head, relative velocity of water at the outlet is reduced.
03. Due to positive centrifugal head, relative velocity of water at the outlet is increases.
04. Discharge does not increase.
04. Discharge increases.
05. Less or no tendency of wheel to race if the turbine speed increases, as the turbine itself adjust the speed.
05. If the turbine speed increases, wheel tends to race as turbine by itself can not adjust the speed.
06. Effective and easy speed control is possible.
06. Speed control is difficult.
07. Used for large output and suitable for medium and high heads.
07. Used for low and medium heads.
08. Used often in power plants.
08. Obsolete.



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