Impulse turbine and reaction turbine working principle

In this article, we see about the Impulse turbine and reaction turbine working principle

Introduction

The steam turbine is widely used as a prime mover in power plants, refineries, petrochemical plants, food processing plants etc.,

The advantages of the Steam turbine are

• Ability to utilize high pressure and high-temperature steam.
• High efficiency.
• Higher rotational speed.
• High capacity/weight ratio.
• Smooth, nearly vibration-free operation.
• No internal lubrication.
• Oil-free exhaust steam.
• Can be built in small or very large units (up to 1200 MW).

The Disadvantages of the Steam turbine are

• For slow speed application reduction gears are required.
• The steam turbine cannot be made reversible.
• The efficiency of small simple steam turbines is poor.

Steam Turbine Operation Principles

Based on operating principle the steam turbine is classified as two types

1. Impulse turbine
2. 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.

The steam passes through the stationary nozzles and is directed at high velocity against the rotor buckets causing the rotor to rotate at high speed.

Events take place in the nozzle

• 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 is converted into kinetic energy. They are two types of nozzles used in the steam turbine. They are

1. Convergent nozzles
2. Convergent-divergent nozzles

Convergent Nozzles

They  are used for smaller pressure drops where the minimum exit pressure is 0.577 x the inlet pressure (the critical pressure for nozzles.) If the exit pressure is less than 0.577 x inlet pressure, eddy-currents are developed and the exit velocity will be less than calculated.

Convergent-divergent Nozzles

The 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 is converted the steam exit velocity will be 0 m/s. This is not possible but it shows that the rotor blades must bring the steam exit velocity near 0 m/s.

The Impulse Principle

The steam at high pressure enters through a stationary nozzle of steam turbine, as a result, the pressure of the steam is decrease and 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 is changed. The effect of this change in direction of the steam flow will produce an impulse force. This force cause the blade move, thereby the rotor will start rotating.

The force applied to the blade is developed by causing the steam to change the direction of flow (Newton’s 2nd Law – change of momentum). The change of momentum produces the impulse force.

Impulse Turbine Working

In the impulse turbine 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 is the distinguishing feature of the impulse turbine. The pressure at the inlet of the moving blades is the same as the pressure at the outlet of moving blades.

Reaction Turbine Principle

In the 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

A 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 is changed thus producing an impulse force on the moving blades. Also, the steam upon passing through the moving blades again expands and further drops in pressure giving a reaction force to the blades.

This sequence is 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.

The distinguishing feature of the reaction turbine is the fact that the pressure does drop across the moving blades. In other words, there is a pressure difference between the inlet to the moving blades and the outlet from 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. Blade clearances, therefore, must be kept to a minimum.
• Also, due to pressure drop across the moving blades, an unbalanced thrust will be developed upon the rotor and some arrangement must be made to balance this.

Conclusion

The steam turbines are widely used in both power generation and process industries. As we discussed, the impulse and reaction turbines are used in all types of applications. But when compared with the reaction turbine, Impulse types are widely used in industries.