Give examples of impulse machines.

Short Answer:

Impulse machines are hydraulic machines that work by converting the entire pressure energy of the fluid into kinetic energy before it strikes the runner or wheel. In these machines, the fluid jet strikes the bucket or blade, and the change in momentum causes rotation. Examples of impulse machines include the Pelton wheel turbine, Turgo turbine, and Girard turbine.

These machines are generally used in situations where high head and low discharge are available. They operate efficiently at high velocity and are often employed in hydroelectric power plants situated in hilly areas or mountainous regions.

Detailed Explanation :

Impulse Machines

Impulse machines are a type of hydraulic machine in which the energy of the fluid is completely converted into kinetic energy before it comes in contact with the runner. The jet of fluid, usually water, is directed through a nozzle and strikes the vanes or buckets attached to the runner. The force of the jet causes a change in momentum, which results in the rotation of the runner, thereby producing mechanical work.

In impulse machines, the entire pressure energy is converted into velocity head before striking the runner. Hence, there is no change in the pressure of the fluid while it passes over the blades; only its velocity and direction change. This is the key characteristic that differentiates impulse machines from reaction machines, where the pressure energy change occurs within the runner itself.

Working Principle

The working principle of impulse machines is based on Newton’s Second Law of Motion, which states that the rate of change of momentum is directly proportional to the applied force. When a high-speed jet of water strikes the blades or buckets of the turbine, it changes direction, resulting in a change in momentum. This change in momentum produces an impulse force that rotates the runner and performs mechanical work.

The nozzle plays an important role in converting the pressure energy of the water into kinetic energy. The high-velocity jet from the nozzle impinges on the runner blades, transferring its energy efficiently. The pressure of the fluid remains constant during this process, and only the velocity is altered.

Examples of Impulse Machines

  1. Pelton Wheel Turbine:
    The Pelton wheel is the most common and classic example of an impulse machine. It is used for high-head and low-discharge applications. The turbine consists of a wheel with double-cupped buckets mounted around its circumference. A high-velocity jet of water from the nozzle strikes the buckets, causing the wheel to rotate. The Pelton wheel is highly efficient in converting the energy of water at high heads into mechanical energy and is often used in hydroelectric power plants in mountainous regions.
  2. Turgo Turbine:
    The Turgo turbine is another type of impulse turbine, similar to the Pelton wheel but designed for medium head and discharge. In this turbine, the jet strikes the runner at an angle (typically around 20°), allowing water to pass through the runner instead of deflecting backward. This design allows higher flow rates and speeds compared to the Pelton wheel, making the Turgo turbine suitable for smaller hydroelectric installations.
  3. Girard Turbine:
    The Girard turbine is an early form of impulse turbine developed for moderate head applications. It uses a single jet of water directed on curved vanes mounted on the runner. Though less common today, the Girard turbine served as a base for the development of other modern impulse turbines.

Characteristics of Impulse Machines

  • In impulse machines, water jet velocity is very high because all pressure energy is converted into kinetic energy before striking the runner.
  • The pressure of the fluid remains constant while it passes over the blades.
  • The casing of the turbine does not perform any hydraulic function but merely serves to prevent water from splashing.
  • They are suitable for high-head, low-discharge conditions.
  • The efficiency of impulse machines remains high even with varying loads.
  • Flow control is achieved by adjusting the nozzle or spear mechanism.

Applications

Impulse machines are commonly used in:

  • High-head hydroelectric power stations.
  • Mountainous and hilly areas where water is available at high pressure but in small quantity.
  • Remote areas where compact and simple design turbines like the Pelton wheel can be easily installed.
  • Systems requiring high rotational speed with small discharge.

The simplicity and robustness of impulse machines make them reliable for continuous operation under challenging conditions.

Advantages

  • High efficiency at high head and low discharge.
  • Simple construction and easy maintenance.
  • Stable performance under variable load conditions.
  • No need for a pressure-tight casing.
  • Suitable for high rotational speeds.

Disadvantages

  • Not suitable for low-head applications.
  • Efficiency decreases with high discharge or fluctuating water levels.
  • Requires careful alignment and maintenance of the nozzle system.
  • Large installation height is needed due to high head requirements.
Conclusion

Impulse machines play a vital role in hydroelectric power generation, especially in areas where water is available under high pressure but in limited quantity. Machines like the Pelton wheel, Turgo, and Girard turbines effectively convert the kinetic energy of high-velocity water jets into mechanical energy with minimal energy loss. Their simple design, high efficiency, and reliability make them a preferred choice for high-head power plants and remote installations.