Classification of Hydraulic Turbine | Civil Engineering Optional Notes for UPSC PDF Download

According to the type of energy:

• Impulse turbine
• Reaction turbine

If the energy available at the inlet of the turbine is only kinetic energy, it's known as an impulse turbine (e.g., Pelton wheel). An impulse turbine operates under atmospheric pressure. If water possesses both kinetic and pressure energy at the inlet, it's called a reaction turbine (e.g., Francis turbine, Kaplan turbine).

According to the direction of water flow in the runner:

• Tangential flow turbine - Water strikes tangentially (e.g., Pelton wheel)
• Radial flow turbine - Water flows radially (inward radial like Thomson turbine, or outward radial like Fourneyron turbine)
• Axial flow turbine - Water flows parallel to the axis (e.g., Kaplan turbine)
• Mixed flow turbine - Water enters radially but leaves parallel to the axis (Modern Francis turbine)

According to head and water quantity:

• High head turbine - High head, small water quantity (Pelton)
• Medium head turbine - Medium head, relatively large water quantity (Francis)
• Low head turbine - Low head, large water quantity (Kaplan)

According to shaft position:

• Horizontal turbines - Horizontal shaft (Pelton wheel)
• Vertical turbines - Vertical shafts (Kaplan)

According to specific speed:

• Low specific speed turbines (Pelton)
• Medium specific speed turbines (Francis)
• High specific speed turbines (Kaplan)

Impulse Turbine

Impulse turbines are characterized by the pressure at the nozzle exit being atmospheric, with pressure remaining constant along the moving vanes except for minor losses. The thrust experienced by the moving vane is due to the change in momentum direction. The Pelton wheel, a common type of hydraulic impulse turbine, operates effectively at heads ranging from 150 to 1500 meters.

Classification of Hydraulic Turbines

The Pelton wheel, also known as a Pelton turbine, is a type of impulse turbine. Below is a discussion of its essential parts and working mechanism.

• Runner and Buckets: The runner is a circular disc with evenly spaced semi-ellipsoidal buckets around its perimeter. Each bucket is divided into two hemispherical cups by a sharp-edged ridge called a splitter, which prevents axial and end thrust on bearings. Buckets are usually bolted to the runner's periphery, allowing for economical replacement if damaged, although in some cases, they are cast as a single piece with the wheel.
• Nozzle and Guide Mechanism: The nozzle, located at the end of the penstock near the turbine, features a conical needle or spear to regulate water flow and control the runner's speed. This spear is operated manually via a hand wheel for small units or automatically by a governing mechanism for larger units.
• Hydraulic Brake: To quickly bring the turbine to rest, a small brake nozzle is used. When the inlet valve is closed, the runner takes time to stop due to inertia. The brake nozzle directs a water jet at the back of the buckets, acting as a brake to halt the runner swiftly.
• Casing: The casing, made of cast iron or fabricated steel plates, serves multiple purposes:
  • Prevents water splashing.
  • Directs water to the tail race.
  • Acts as a safety cover against accidents.

Working of a Pelton Wheel

Water is delivered to the powerhouse from the head race through penstocks. The nozzle at the penstock's end directs a high-velocity water jet into the buckets. The water jets impinge tangentially on the buckets, causing the wheel to rotate and produce mechanical work. An electric generator coupled to the runner shaft converts this mechanical energy into electrical power. After leaving the turbine wheel, water falls into the tail race, with the Pelton wheel positioned above to avoid splashing.

Reaction Turbine

Reaction turbines are a type of impulse-reaction prime mover, where the rotor of moving vanes is surrounded by a stator of guide vanes. The guide vanes function as nozzles, allowing partial expansion of pressure into kinetic energy. Further expansion occurs as the fluid moves over the moving vanes, creating thrust through both momentum change (impulse) and pressure action (reaction). Since water is admitted around the entire wheel periphery and pressure varies across the moving vanes, reaction turbines are completely sealed from the atmosphere. Common reaction turbines include the Francis Turbine for medium heads (20-200 m) and the Kaplan Turbine for low heads (2-3 m).

Francis Turbine

The Francis turbine is an inward flow reaction turbine with radial discharge at the outlet. In modern designs, water enters the runner radially towards the center and exits axially, making it a mixed flow type turbine.

Working of a Francis Turbine

1. Water Delivery: Water is delivered from the reservoir to the turbine through penstocks, entering the scroll casing.
2. Scroll Casing: This casing distributes water evenly around the turbine runner's circumference.
3. Stay Ring: Water passes through the stay ring, which directs it to the guide vanes.
4. Guide Vanes: These regulate the water quantity supplied to the runner and allow smooth, shock-free flow due to their airfoil shape.
5. Runner Entry: Water enters the runner at low velocity and high pressure.
6. Flow and Energy Conversion: As water flows through the runner, its direction changes from axial to radial, converting pressure energy to kinetic energy, which rotates the runner at high speed.
7. Energy Transmission: This torque is transmitted to the generator coupled to the runner shaft.
8. Exit: Water exits through the runner into the tail race via a draft tube.

Axial Flow Reaction Turbines

Axial flow turbines have water flowing parallel to the shaft's rotation axis, with a vertical shaft. The lower end of the shaft, known as the hub or boss, has blades fixed on it, acting as the runner.

Types of axial flow reaction turbines include:

1. Propeller Turbine: The blades are fixed and not adjustable, integrated with the hub.
2. Kaplan Turbine: The blades on the hub are adjustable.

These turbines are suitable for low heads and require a large quantity of water to generate power.

Main Components of a Kaplan Turbine

1. Scroll Casing
2. Guide Vanes Mechanism
3. Hub with Blades
4. Draft Tube