What is an impulse turbine?

Short Answer:

An impulse turbine is a type of steam turbine in which the steam expands completely in the stationary nozzles before striking the moving blades. The high-velocity steam jet hits the blades, changes its direction, and produces a force that rotates the turbine shaft. In this turbine, steam pressure remains constant while passing over the moving blades.

Impulse turbines are mainly used for high-speed applications and are simple in design. They work on the principle of Newton’s second law of motion, where a change in momentum of steam causes a reactive force on the blades, producing mechanical energy.

Detailed Explanation :

Impulse Turbine

An impulse turbine is one of the main types of steam turbines that converts the thermal energy of high-pressure steam into mechanical energy by using the kinetic energy of a steam jet. It is widely used in power generation and mechanical drive applications where high speed and simplicity are required. The working of an impulse turbine is based on the principle that when a jet of fluid with high velocity strikes a curved surface, it exerts a force due to the change in momentum, and this force can be used to produce rotation.

Working Principle

The impulse turbine operates on the principle of Newton’s second law of motion, which states that the rate of change of momentum is directly proportional to the applied force. In this turbine, the entire expansion of steam occurs in the stationary nozzles. These nozzles convert the high-pressure, low-velocity steam into a high-velocity jet by dropping its pressure and increasing its kinetic energy.

When the steam jet hits the moving blades, it changes direction, and this change in momentum exerts a force on the blades. This force causes the rotor to spin, converting kinetic energy into mechanical energy. After striking the blades, the steam leaves the turbine at low velocity but with the same pressure.

Main Components of an Impulse Turbine

  1. Nozzles:
    These are stationary passages that convert high-pressure steam into a high-velocity jet. The pressure drop of steam occurs completely in the nozzles.
  2. Blades (Buckets):
    The blades are mounted on the turbine rotor and designed in curved shapes to efficiently deflect the steam jet. The change in steam direction produces an impulse force that rotates the rotor.
  3. Rotor or Shaft:
    It carries the moving blades and converts the impulse force into rotational motion.
  4. Casing:
    The outer cover that encloses the turbine and prevents steam leakage.
  5. Nozzle Box:
    It supplies steam to the nozzles and helps in directing the steam flow properly.
  6. Bearings:
    These support the rotating shaft and reduce friction for smooth operation.

Working Process Step-by-Step

  1. Steam Supply:
    High-pressure steam from the boiler enters the nozzle box of the turbine.
  2. Expansion in Nozzle:
    The steam expands completely in the nozzle, where its pressure energy is converted into velocity energy.
  3. Impact on Blades:
    The high-velocity steam jet hits the curved blades mounted on the rotor.
  4. Change of Direction:
    The direction of steam changes while striking the blades, producing an impulse force according to Newton’s laws.
  5. Rotation of Rotor:
    The generated force causes the rotor to rotate and deliver mechanical work to the output shaft.
  6. Exhaust of Steam:
    The steam leaves the moving blades at low velocity and nearly constant pressure.

Types of Impulse Turbine

Impulse turbines are further divided into two main types:

  1. Simple Impulse Turbine (De Laval Turbine):
    In this type, steam expands completely in a single nozzle and impinges on a single row of moving blades. It is used for small power outputs and simple applications.
  2. Velocity Compounded Impulse Turbine (Curtis Turbine):
    In this type, multiple stages of blades are used to gradually absorb the kinetic energy of steam. This design helps in reducing rotor speed and improving efficiency for large power outputs.

Features of Impulse Turbine

  • Steam expansion occurs only in the nozzles.
  • The moving blades experience no pressure drop.
  • High-velocity steam jets are used for rotation.
  • The design is simple and easy to maintain.
  • Can operate under very high pressure and temperature conditions.

Advantages of Impulse Turbine

  • Simple design and construction.
  • Easy maintenance and operation.
  • Suitable for high-speed applications.
  • Capable of handling high-pressure steam.
  • Works efficiently in small and large power ranges.

Disadvantages of Impulse Turbine

  • Efficiency decreases at partial loads.
  • High steam velocity causes erosion of blades over time.
  • More noise due to high-speed jets.
  • Requires precise nozzle and blade design for effective operation.

Applications of Impulse Turbine

  • Used in De Laval and Curtis turbine designs.
  • Commonly used in thermal power plants for electricity generation.
  • Suitable for marine propulsion systems.
  • Used in industrial drives and mechanical applications requiring high speed.
Conclusion

In conclusion, an impulse turbine is a simple and efficient machine that converts the kinetic energy of high-velocity steam into mechanical energy. It works by the impulse force produced when the direction of steam flow is changed by the blades. Since the pressure drop occurs entirely in the nozzles, the turbine blades only experience velocity forces. Impulse turbines are preferred for applications where high-speed operation, reliability, and compact design are essential. They form the foundation for more complex turbine systems used in modern power generation.