Hydraulic Turbines: Classifications and Operating Principles
Introduction
Hydraulic turbines are turbomachines that extract energy from moving water and convert it into mechanical work, which is then used to spin generators in hydroelectric power plants. Choosing the right turbine depends on the available hydraulic head and flow rate at the site.
Turbine Classification
Hydraulic turbines are broadly classified into two main categories based on their operating principle:
- Impulse Turbines: The potential head of the water is fully converted into kinetic energy via a nozzle before hitting the runner. The runner operates at atmospheric pressure. The most common type is the Pelton Wheel, which is highly efficient for high heads and low flow rates.
- Reaction Turbines: The runner is completely submerged in water and utilizes both pressure and kinetic energy. The water pressure drops as it moves through the turbine. Key types include:
- Francis Turbine: A mixed-flow design suitable for medium heads and medium flow rates. It is the most widely used turbine in the world.
- Kaplan Turbine: An axial-flow propeller turbine with adjustable blades, optimal for low heads and high flow rates.
Performance Comparison
| Turbine Type | Operating Type | Hydraulic Head Range | Specific Speed Range |
|---|---|---|---|
| Pelton | Impulse | High (300m – 1500m) | Low (10 – 50) |
| Francis | Reaction | Medium (30m – 300m) | Medium (50 – 250) |
| Kaplan | Reaction | Low (2m – 40m) | High (250 – 800) |
Cavitation in Reaction Turbines
A major operational challenge for reaction turbines is cavitation. If the local pressure drops below the vapor pressure of water, vapor bubbles form. When these bubbles move to high-pressure zones, they collapse violently, causing pitting, structural vibrations, and loss of efficiency. Proper installation elevation and material coatings are critical to avoid cavitation damage.