What is an impulse turbine?

Short Answer:

An impulse turbine is a type of turbine that converts the kinetic energy of a high-velocity water jet into mechanical energy by striking the turbine blades or buckets. The entire pressure energy of water is first converted into kinetic energy using a nozzle before it hits the runner blades. The change in momentum of water while striking the blades causes the runner to rotate and produce mechanical power.

Impulse turbines are mainly used for high-head and low-discharge conditions, such as in mountainous regions. The most common example of an impulse turbine is the Pelton wheel turbine.

Detailed Explanation:

Impulse Turbine

An impulse turbine is a hydraulic machine that works on the principle of impulse, where a jet of water with high kinetic energy strikes the blades (or buckets) of the turbine and changes its direction. The resulting change in momentum exerts a force on the blades, which makes the runner rotate and deliver mechanical energy to the shaft. This mechanical energy can then be used to drive an electric generator or other mechanical equipment.

Impulse turbines are designed to operate under high head and low flow rate conditions. They are used in hydroelectric power plants where water from a reservoir flows through a nozzle and is directed onto the turbine blades at very high speed. The turbine converts this kinetic energy into rotational energy efficiently.

Working Principle

The working principle of an impulse turbine is based on Newton’s second law of motion, which states that the rate of change of momentum is directly proportional to the force applied. In an impulse turbine, water at high pressure is passed through a nozzle that converts all its pressure energy into kinetic energy. The resulting high-speed jet of water hits the buckets or blades mounted on the runner.

When the water jet strikes the curved surface of the blades, it changes direction and exerts an impulsive force on them. This impulse causes the runner to rotate. The rotation of the runner shaft is then used to generate electricity in power plants. Importantly, the pressure of water remains constant while it flows over the blades — only the velocity changes.

Construction of Impulse Turbine

The main components of an impulse turbine are:

  1. Nozzle:
    The nozzle converts the available pressure energy of water into high-velocity kinetic energy. The nozzle is usually fitted with a needle or spear to control the flow of water jet.
  2. Runner and Buckets (or Blades):
    The runner is a circular disc fitted with a number of double-cupped buckets around its periphery. The water jet strikes these buckets tangentially, causing the runner to rotate. The bucket design helps in changing the direction of the jet efficiently, ensuring maximum momentum transfer.
  3. Casing:
    The casing prevents the splashing of water and helps direct the discharged water to the tailrace. It also provides safety and protection to the turbine components.
  4. Spear and Nozzle Control Mechanism:
    This mechanism adjusts the size of the nozzle opening and controls the velocity and discharge of water entering the turbine.
  5. Brake Nozzle:
    When the turbine needs to be stopped, the brake nozzle directs a jet of water in the opposite direction to bring the runner to rest quickly.

Working Process

  1. Water from the reservoir is passed through the penstock pipe under high pressure.
  2. The nozzle at the end of the penstock converts this pressure energy into kinetic energy.
  3. The high-speed jet of water strikes the runner buckets tangentially.
  4. The jet changes direction while hitting the blades, exerting an impulse force that makes the runner rotate.
  5. After striking the blades, the water falls into the tailrace at atmospheric pressure.
  6. The mechanical energy from the rotating shaft is used to drive an electrical generator.

The process is highly efficient because all pressure energy is converted into kinetic energy before striking the blades, and water pressure remains constant throughout the interaction.

Characteristics of Impulse Turbine

  • Water pressure remains constant while flowing over the blades.
  • Operates under atmospheric pressure inside the casing.
  • The flow of water is tangential to the runner.
  • Suitable for high-head, low-discharge locations.
  • The turbine works efficiently even with a single water jet (in small turbines) or multiple jets (in large turbines).
  • It has a simple design and is easy to maintain.

Examples of Impulse Turbines

  • Pelton Wheel Turbine: Used for very high heads (above 250 meters).
  • Turgo Impulse Turbine: Suitable for medium head and moderate flow conditions.
  • Cross-flow Turbine (Banki Turbine): Used for small-scale power generation and low-cost installations.

These turbines are commonly used in mountainous regions where water falls from great heights with high potential energy.

Conclusion:

An impulse turbine is an efficient machine used to convert the kinetic energy of high-speed water jets into mechanical energy. It operates entirely under atmospheric pressure and is best suited for high-head, low-flow situations. The key idea behind its operation is the impulse force generated when the water jet strikes and deflects off the turbine blades. Pelton wheels and similar designs are practical examples widely used in hydroelectric power stations. Impulse turbines play a vital role in energy generation, especially in locations where natural water heads are available.