Short Answer:

A draft tube is a pipe or conduit fitted at the outlet of a reaction turbine. Its main function is to discharge the used water from the runner to the tailrace and to convert the remaining kinetic energy of water into useful pressure energy. It helps in improving the efficiency of the turbine by reducing velocity losses and allowing smooth water flow.

In simple terms, a draft tube recovers the energy of the water leaving the turbine runner and maintains a low-pressure area at the runner exit. This low pressure increases the effective head acting on the turbine, resulting in higher output and better performance of the hydraulic system.

Detailed Explanation :

Draft tube

A draft tube is an essential component of reaction turbines such as the Francis and Kaplan turbines. It is a gradually expanding pipe fitted at the outlet of the turbine runner, which carries the water discharged from the runner to the tailrace. The draft tube performs two main functions: it allows the runner to be placed above the tailrace level without loss of head, and it recovers a part of the kinetic energy of the discharged water by converting it into pressure energy. This improves the overall efficiency of the turbine.

In reaction turbines, water does not leave the runner freely as in impulse turbines; it exits with some velocity and pressure. The draft tube is designed to handle this flow by gradually expanding its cross-section so that the velocity of the water decreases while its pressure increases. According to Bernoulli’s principle, this conversion of velocity head into pressure head helps recover some of the lost energy, which would otherwise be wasted.

The construction of a draft tube depends on the type of turbine and the head under which it operates. It is usually made of steel, cast iron, or reinforced concrete. The inner surface of the draft tube is smooth to minimize frictional losses, and its shape is designed to allow gradual expansion of flow. If the expansion angle is too large, it can cause flow separation and energy losses due to eddies or turbulence. Hence, the typical expansion angle of a draft tube ranges between 5° to 10°.

The functions of the draft tube are as follows:

1. It allows the turbine to be installed above the tailrace without loss of head.
2. It converts a large portion of the kinetic energy of the water leaving the runner into pressure energy.
3. It ensures smooth discharge of water into the tailrace, preventing splashing or air entry.
4. It maintains a vacuum at the runner exit, which increases the effective working head on the turbine.
5. It supports the turbine structure by connecting the runner casing to the tailrace.

There are several types of draft tubes commonly used in turbines:

1. Straight Conical Draft Tube:
   This type consists of a straight cone that gradually expands from the runner exit to the tailrace. It is simple in design and easy to construct, mainly used for small turbines where the efficiency requirement is moderate.
2. Elbow Type Draft Tube:
   The elbow type draft tube has a bend or curve that allows horizontal discharge of water. It is commonly used in Kaplan turbines and low-head installations where vertical space is limited. The curved design helps direct water smoothly into the tailrace.
3. Moody Spreading Draft Tube:
   This type has a circular cross-section near the runner and a rectangular outlet at the bottom. It provides better energy recovery and smooth flow distribution at the outlet.
4. Bell-Mouthed or Hydraulically Designed Draft Tube:
   This type of draft tube has a bell-shaped inlet that minimizes entry losses. It provides very efficient energy conversion and smooth water flow, commonly used in large hydraulic power plants.

The working principle of a draft tube is based on Bernoulli’s theorem. When water leaves the runner, it possesses both kinetic energy and some pressure energy. As it flows through the expanding section of the draft tube, the velocity of the water decreases, and this reduction in velocity causes a rise in static pressure. The increase in pressure energy helps to recover part of the kinetic energy that would otherwise be lost at the runner exit. This recovered energy effectively increases the total head available to the turbine.

For a draft tube to function efficiently, it must be completely filled with water at all times. Any air leakage or cavitation inside the tube can disturb the flow and reduce turbine efficiency. Cavitation occurs when the pressure inside the draft tube drops below the vapor pressure of water, leading to the formation of vapor bubbles. Therefore, proper design and maintenance are essential to avoid such problems.

The efficiency of a draft tube, also known as the draft tube efficiency, is defined as the ratio of the actual increase in pressure head to the kinetic head at the runner exit. A well-designed draft tube can recover up to 80–90% of the kinetic energy, thus significantly improving the turbine’s performance.

In hydroelectric plants, the draft tube not only enhances energy recovery but also supports the mechanical structure by connecting the turbine casing with the tailrace chamber. It ensures a continuous, steady discharge of water, contributing to stable and efficient turbine operation.

Conclusion:

A draft tube is an important part of a reaction turbine that improves efficiency by converting kinetic energy into pressure energy and allowing the turbine to be installed above the tailrace. Its design and shape play a major role in ensuring smooth flow and preventing energy losses. A properly designed draft tube helps in achieving maximum energy recovery and long-term efficient operation of hydraulic turbines.