Question

With a neat diagram explain pelton wheel turbine.

Answer 1

Introduction

The Pelton wheel or Pelton turbine is a type of impulse turbine used in hydroelectric power plants. It is a tangential flow turbine, meaning the water strikes the runner (the rotating part) at a tangent. It is best suited for applications with a very high head (the vertical distance the water falls) and a low flow rate.

The fundamental principle is straightforward: it converts the kinetic energy of a high-velocity jet of water into mechanical energy, which then drives a generator to produce electricity.

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Diagram of a Pelton Wheel Turbine

Here is a neat, labeled diagram showing the main components and the working of a Pelton wheel turbine.

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Main Components and Their Functions

Let's break down the key parts shown in the diagram:

1. Runner and Buckets:
   The Runner is the rotating wheel of the turbine.
   Attached to its periphery are several Buckets. These buckets are shaped like a double hemispherical cup, with a sharp edge in the middle called a splitter.
   * Function: The splitter divides the incoming water jet into two equal streams, which then flow along the smooth inner curve of the bucket. This design is crucial as it deflects the water jet through an angle of 160° to 170°, almost reversing its direction. This large change in the direction (and thus momentum) of the water creates a strong impulsive force on the bucket, causing the runner to rotate.

2. Nozzle and Spear:
   The Nozzle is a conical pipe at the end of the penstock that converts the high-pressure water into a high-velocity jet.
   Inside the nozzle is a movable, cone-shaped needle called a Spear.
   * Function: The spear can be moved forward or backward. Moving it forward reduces the annular area of the nozzle outlet, decreasing the flow rate of water. Moving it backward increases the flow. This mechanism allows for precise control of the water jet's power and helps regulate the turbine's speed and power output efficiently without significant energy loss.

3. Casing:
   This is the outer cover of the turbine.
   Function: Unlike in reaction turbines, the casing of a Pelton wheel is not airtight and contains no pressure. Its main purposes are:

   *   To prevent the splashing of water and avoid accidents.
   *   To guide the water that has struck the buckets down to the **tailrace** (the water channel at the bottom).
   *   To act as a safeguard.
   

4. Penstock:
   This is a large-diameter pipe that carries water from the high-level reservoir (like a dam) down to the turbine nozzle.
   Function: It delivers the water with high potential energy (due to the head) to the nozzle.

5. Braking Jet:
   This is a smaller nozzle that directs a jet of water onto the back of the buckets.
   Function: When the turbine needs to be stopped quickly (e.g., if the electrical load is suddenly removed), the main water jet is shut off by the spear. However, the runner continues to rotate due to inertia. The braking jet is then used to apply a retarding force to bring the runner to a halt quickly and safely.

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Working of a Pelton Wheel Turbine

The operation can be explained in a step-by-step cycle:

1. Water Collection: Water from a high-altitude reservoir is transported through the penstock under high pressure.

2. Jet Formation: At the end of the penstock, the water passes through the nozzle, where its high pressure energy is converted into high kinetic energy, forming a powerful, high-velocity jet of water. The flow rate is controlled by the spear.

3. Impulse Action: This high-velocity jet is directed tangentially towards the buckets on the runner.

4. Momentum Transfer: The jet strikes the splitter of a bucket, which divides it into two streams. These streams glide along the inner smooth surface of the bucket and are deflected by almost 180°.

5. Rotation: According to Newton's second law, this change in the momentum of the water exerts an impulsive force on the bucket. This force creates a torque on the runner, causing it to rotate at a high speed.

6. Power Generation: The runner is connected to a shaft, which is coupled to an electric generator. The mechanical energy of the rotating runner is converted into electrical energy by the generator.

7. Water Discharge: After imparting its energy to the buckets, the water has very low velocity and falls due to gravity into the tailrace at the bottom of the casing, from where it is discharged. The entire process occurs at atmospheric pressure inside the casing.

Key Characteristics & Applications

• Type: Impulse Turbine.
• Flow: Tangential Flow.
• Head: High Head (typically > 250 meters).
• Discharge: Low Flow Rate.
• Efficiency: Very high, often exceeding 90%.
• Application: Primarily used in hydroelectric power plants situated in mountainous regions where a large difference in water level (head) is available.

Answer 2

Explanation (Working):
The water stored at high head is made to flow through the penstock and reaches the nozzle
of the Pelton turbine.
The nozzle increases the K.E. of the water and directs the water in the form of jet.
The jet of water from the nozzle strikes the buckets (vanes) of the runner. This made the
runner to rotate at very high speed.
The quantity of water striking the vanes or buckets is controlled by the needle valve
present inside the nozzle.
The generator is attached to the shaft of the runner which converts the mechanical energy
( i.e. rotational energy) of the runner into electrical energy.