Comparison of Impulse and Reaction Turbines:
Impulse Turbine
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Reaction turbine
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01. Energy available at the
inlet is in the form of kinetic energy
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01. Energy available at the
inlet is in the form of kinetic and pressure energy
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02. Pressure acting buckets
is constant and atmospheric throughout the action of water on the runner till
it is discharged to the tail race.
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02. Water enters the runner
with excess pressure and hence both pressure and velocity of water change as
water passes through the runner.
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03. Water does not fill the
vanes and casings and air has free access to the buckets.
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03. Water fills completely
the vanes and buckets throughout the turbine operation.
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04. Casing is used only to
protect water from splashing and to guide water to the tail.
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04. Casing is essential to
send the unit from atmospheric condition as the turbine inlet pressure is
much higher than the outlet pressure.
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05. Turbine unit is installed
above the tail race level.
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05. Turbine unit is
completely submerged in water bellow the tail race level.
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06. Flow regulation does not
result in possible loss.
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06. Flow regulation results
in possible loss.
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07. As water passes over the
guide vanes, its relative velocity reduces slightly or remains the same.
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07. As water glides over the
vanes, its relative velocity increases due to continuous drop in pressure
during flow through the blade passages.
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Comparison of Impulse turbine and Impulse-reaction turbine:
Impulse
turbine
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Impulse
reaction turbine
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01. Steam expands completely in the nozzle.
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01. Steam expands partially in the moving blades and partially in the
fixed blades.
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02. Pressure of steam remains constant during its flow through the
blade passages.
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02. Pressure of steam is not constant during its flow through moving
and fixed blade passages.
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03. Steam flow over the blades is uniform due to symmetrical blade
profile.
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03. Steam flow over the blades varies due to aerofoil section
(unsymmetrical) blade profile.
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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.
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04. Since pressure drop occurs partially in the fixed blades, steam
velocity and rotor speed are not high. Hence stresses developed are less.
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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
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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.
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06. As pressure drop in each stage is large, number of stages required
is less. Hence turbine is compact.
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06. Due to small pressure drop in each stage, it requires large number
of stages. Hence size of turbine is bigger.
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07. Part load efficiency of turbine is poor.
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07. Part time efficiency of the turbine is good due to more efficient
blades.
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08. Speed of turbine is very high and hence requires compounding.
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08. Speed of turbine is low and hence does not require compounding.
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09. Turbine is compact and hence occupies less space.
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09. Turbine is bigger and hence requires more space.
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10. It is installed in small capacity power plants.
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10. It is generally used in medium and large capacity power plants.
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Inward flow turbine vs. Outward flow Reaction Turbine:
Inward
Flow Reaction Turbine
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Outward
Flow Reaction turbine
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01. Water flows the outer periphery of wheel towards the wheel centre
and discharges at the inner periphery.
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01. Water enters the wheel centre (inner periphery) and flows outward
and discharges at the outer periphery.
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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
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02. Water enters the runner with excess pressure and hence both
pressure and velocity of water change as water passes through the runner.
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03. Due to negative centrifugal head, relative velocity of water at the
outlet is reduced.
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03. Due to positive centrifugal head, relative velocity of water at the
outlet is increases.
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04. Discharge does not increase.
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04. Discharge increases.
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05. Less or no tendency of wheel to race if the turbine speed increases,
as the turbine itself adjust the speed.
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05. If the turbine speed increases, wheel tends to race as turbine by
itself can not adjust the speed.
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06. Effective and easy speed control is possible.
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06. Speed control is difficult.
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07. Used for large output and suitable for medium and high heads.
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07. Used for low and medium heads.
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08. Used often in power plants.
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08. Obsolete.
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