What is draft tube?

Short Answer:

A draft tube is a pipe or passage that connects the runner outlet of a reaction turbine to the tailrace, which is the water channel at the exit of the turbine. Its main function is to allow the water to discharge efficiently and recover part of the kinetic energy of the flowing water. The draft tube helps in maintaining a low pressure at the runner exit and increases the overall efficiency of the turbine.

The draft tube is designed with a gradually increasing cross-section to reduce the velocity of water before it leaves the turbine. It allows the turbine to be placed above the tailrace level without losing head, thus preventing cavitation and ensuring smooth discharge of water. It is an important component in turbines like Francis and Kaplan turbines.

Detailed Explanation :

Draft Tube

A draft tube is a vital hydraulic component used in reaction turbines. It is a conical or elbow-shaped pipe fitted at the outlet of the turbine runner, which discharges the water into the tailrace. The main objective of the draft tube is to convert a part of the kinetic energy of the discharged water into pressure energy, thereby improving the efficiency of the turbine. Since reaction turbines operate with water under pressure, the draft tube ensures that the pressure at the runner outlet remains below atmospheric pressure to maintain a continuous flow of water through the turbine.

The draft tube allows the turbine to be installed above the tailrace level, which makes maintenance easier and helps to avoid submerging the turbine components under water. This arrangement, however, would normally result in loss of head, but the draft tube compensates for this by creating a suction head that draws water upward, effectively recovering part of the lost head.

Functions of Draft Tube

  1. Energy Recovery:
    The draft tube converts part of the velocity energy of water leaving the runner into pressure energy. This helps in utilizing most of the water’s energy, increasing the total efficiency of the turbine.
  2. Allowing Turbine Elevation:
    It enables the turbine to be positioned above the tailrace level. This simplifies construction and maintenance while maintaining effective energy conversion.
  3. Pressure Maintenance:
    It maintains low pressure at the runner exit, ensuring a smooth flow of water through the runner blades.
  4. Prevention of Cavitation:
    Cavitation occurs when the local pressure falls below vapor pressure. The draft tube is designed to reduce this possibility by maintaining sufficient pressure at the runner outlet.
  5. Efficient Water Discharge:
    The shape of the draft tube ensures that water is discharged smoothly into the tailrace with minimal energy loss.

Types of Draft Tubes

  1. Straight Conical Draft Tube:
    This type has a simple conical shape with a gradually expanding cross-section. It is suitable for vertical shaft turbines like the Francis turbine. The expansion angle is usually kept between 5° and 8° to minimize energy losses.
  2. Elbow Type Draft Tube:
    It is bent in shape and mostly used in Kaplan turbines or propeller turbines where the runner is located close to the tailrace. It has a horizontal discharge and is efficient in limited height conditions.
  3. Moody Spreading Tube:
    This type is designed with a central core and is used to achieve better energy recovery and flow distribution. It is commonly applied in large-capacity turbines.
  4. Bell-Mouth or Circular Type:
    It has a wide opening at the outlet and is used in turbines that require smooth flow discharge and reduced water turbulence.

Working Principle of Draft Tube

When water flows out of the turbine runner, it still has considerable velocity. The draft tube, having an expanding area, allows the velocity of water to gradually decrease. According to Bernoulli’s principle, when the velocity decreases, the pressure increases. This increase in pressure energy helps to recover part of the kinetic energy lost by the water. The pressure at the exit of the runner becomes lower than the tailrace pressure, creating a suction effect that helps maintain a continuous flow of water through the turbine system.

Advantages of Draft Tube

  • Improves overall turbine efficiency by recovering kinetic energy.
  • Allows installation of turbines above tailrace without loss of head.
  • Prevents cavitation by maintaining proper pressure conditions.
  • Reduces structural problems by simplifying turbine maintenance.
  • Enhances smooth discharge of water into the tailrace.

Design Considerations

  • The draft tube must have a gradually expanding shape to avoid flow separation.
  • The angle of divergence should be small to prevent eddies and losses.
  • The material used must withstand water pressure and corrosion.
  • The length and shape depend on the type of turbine and installation height.
Conclusion

A draft tube is an essential component in reaction turbines like Francis and Kaplan types. It connects the turbine runner to the tailrace and helps convert kinetic energy into pressure energy, improving turbine efficiency. The design and type of draft tube are chosen based on the turbine setup and available head. Its role in preventing cavitation and recovering energy makes it a key element in hydraulic turbine performance.