Short Answer:
A draft tube in a hydraulic turbine is a curved or straight pipe that connects the turbine runner outlet to the tailrace (the channel where water exits). Its main function is to recover the kinetic energy of water leaving the turbine and convert it into useful pressure energy.
By reducing the velocity of the exiting water and increasing its pressure, the draft tube improves the overall efficiency of the turbine. It also allows the turbine to be installed above the tailwater level without energy loss.
Detailed Explanation:
Function of a draft tube in a hydraulic turbine
In hydraulic turbines, water flows through the runner (the rotating part of the turbine) and exits at high velocity. If this water is simply discharged into the tailrace, a significant amount of its energy would be wasted as kinetic energy. To prevent this loss, a draft tube is used.
A draft tube is a gradually expanding pipe that helps in converting the velocity head of water into pressure head, increasing the energy available for power generation. It also allows the turbine casing to be placed at a higher elevation than the tailrace without reducing the pressure at the runner exit below the atmospheric level.
Key Functions of Draft Tube
- Energy recovery
The draft tube slows down the water as it leaves the turbine runner. Due to the gradual expansion of the cross-sectional area, the kinetic energy of water is converted into pressure energy, which enhances the turbine’s efficiency. - Permits higher turbine setting
It allows the turbine to be set above the tailwater level, making inspection and maintenance easier. Without a draft tube, placing the turbine higher would result in energy loss due to suction. - Maintains low pressure at runner exit
The draft tube helps maintain a negative pressure (vacuum) at the runner outlet. This ensures a smooth flow of water and improves the suction head, assisting water movement through the turbine. - Smooth water discharge
It provides a smooth and directed path for water to exit into the tailrace, reducing turbulence and minimizing erosion in the tailwater region. - Structural support
In some designs, the draft tube also provides structural support to the turbine assembly and forms part of the foundation of the turbine pit.
Types of Draft Tubes
- Conical (Straight divergent): Simple in design, gradually widens to reduce water speed.
- Elbow (Bent) type: Used where space is limited, bends horizontally before releasing water.
- Moody spreading type: Includes a combination of bends and expansions for higher recovery.
- Hydraulic bell mouth: Smooth and efficient, commonly used for vertical shaft turbines.
The choice depends on site layout, type of turbine, and efficiency requirements.
Importance in Civil Engineering Projects
In hydroelectric projects, the use of a well-designed draft tube is essential. It helps engineers:
- Improve turbine output
- Reduce excavation depth
- Avoid air entrapment
- Maintain cost-effective plant layout
Calculations involving draft tube efficiency, losses, and pressure head are part of turbine design in civil engineering hydraulics.
Conclusion:
A draft tube plays a vital role in increasing the efficiency of a hydraulic turbine by converting leftover kinetic energy into pressure energy. It also allows the turbine to be installed above the tailrace, facilitating better layout and maintenance. Proper design and positioning of the draft tube are essential for efficient and smooth operation of hydroelectric power stations.