Introduction
In this article, we learn about the Impulse Turbine and Reaction Turbine-Principle and Working.
The steam turbines are widely used as prime mover in power plants, refineries, petrochemical industries, food processing industries.
To read about the difference between Impulse turbine and Reaction Turbine, Click Here
Steam Turbine Operation Principles
Based on operating principle the steam turbines are classified into two types
- Impulse turbine
- Reaction turbine
Impulse Turbine
In principle, the impulse steam turbine consists of a casing containing stationary steam nozzles and a rotor with moving or rotating buckets.
When the steam passes through the stationary nozzles and it’s direct toward rotor buckets with high velocity. Thereby rotor buckets start to rotate at high speed.
Events take place in the nozzle
The following event takes place in the nozzles are
- The steam pressure decreases.
- The enthalpy of the steam decreases.
- The steam velocity increases
- The volume of the steam increases.
In nozzles, the pressure energy of the steam converted into kinetic energy. They are two types of nozzles used in the steam turbine. They are
- Convergent nozzles
- Convergent -divergent nozzles
The Convergent Nozzles
This type of nozzle used for smaller pressure drops, where the minimum exit pressure of the turbine equals 0.577 x the inlet pressure (the critical pressure for nozzles), This type of nozzle is used for small steam turbines
The main disadvantage of this nozzle, the exit pressure of the steam usually less than the calculated value of “0.577 x inlet pressure”, due to the formation of eddy-currents. The eddy current losses are overcome in “Convergent-Divergent nozzles”
The Convergent-divergent Nozzles
In convergent-divergent nozzles prevent eddy currents and the calculated velocity will be obtained even at large pressure drops.
The purpose of the bucket or moving blade on the rotor is to convert the kinetic energy of the steam into mechanical energy. If all kinetic energy converted into pressure energy, the steam exit velocity will be 0 m/s. Practically it’s not possible but it shows that the rotor blades must bring the steam exit velocity near 0 m/s.
Principle Of Impulse Turbine
The steam at high pressure enters through a stationary nozzle of a steam turbine, as a result, the pressure of the steam decreases with an increase in steam velocity. As a result of increased steam velocity steam pass through the nozzle in the form of a high-speed jet. This high-velocity steam hit the properly shaped turbine blade, as a result, the steam flow direction changed. The effect of this change in direction of the steam flow will produce an impulse force. This force causes the blade to move, thereby the rotor will start to rotate.
As per Newton’s 2nd Law of motion (change of momentum), The change of direction of steam flow in the blade produces the impulse force, this force tends to rotate the turbine rotor. Hence it’s called “Impulse Turbine”
Impulse Turbine Working
In an impulse turbine, the steam pressure drops and the velocity increases as the steam passes through the nozzles. When the steam passes through the moving blades the velocity drops but the pressure remains the same.
The fact that the pressure does not drop across the moving blades, hence the pressure at the inlet of the moving blade equals the pressure at the outlet of moving blades.
Reaction Turbine Principle
In the case of a reaction turbine, the moving blades of a turbine are shaped in such a way that the steam expands and drops in pressure as it passes through them. As a result of the pressure decrease in the moving blade, a reaction force will be produced. This force will make the blades to rotate.
Reaction Turbine Working
The reaction turbine has rows of fixed blades alternating with rows of moving blades. The steam expands first in the stationary or fixed blades where it gains some velocity as it drops in pressure. Then enters the moving blades where its direction of flow changed thus producing an impulse force on the moving blades. In addition, the steam passing through the moving blades will expand and further drops in pressure giving a reaction force to the blades.
This sequence repeated as the steam passes through additional rows of fixed and moving blades.
Note that the steam pressure drops across both the fixed and the moving blades while the absolute velocity rises in the fixed blades and drops in the moving blades.
Special Aspects of Reaction Turbines
- There is a difference in pressure across the moving blades. The steam will, therefore, tend to leak around the periphery of the blades instead of passing through them. Hence the blade clearances as to maintain as minimum as possible.
- Due to the pressure drop across the moving blades, an unbalanced thrust will be developed upon the rotor and some arrangement must be made to balance the unbalanced force.
- The reaction turbine required a larger plot area, due to the larger size of the turbine.
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