Understanding Draft Tubes in Hydro Turbines

A draft tube is a crucial component used in hydroelectric power plants. It is essentially a diverging tube installed at the exit of the runner of a turbine.

The primary function of a draft tube is to recover kinetic energy from the water as it leaves the turbine runner and convert it into pressure energy. This recovery helps to increase the effective head across the turbine, thereby improving its overall efficiency.

Let's consider the types of turbines mentioned:

• Impulse Turbines: These turbines, like the Pelton wheel, operate based on the principle of impulse. A high-velocity jet of water strikes the buckets of the runner, and the entire available pressure energy is converted into kinetic energy before the water enters the runner. The runner operates at atmospheric pressure, and the water discharges into the tailrace also at or near atmospheric pressure. Since the discharge occurs at atmospheric pressure, there is no significant pressure head to recover using a draft tube.
• Reaction Turbines: These turbines, such as Francis, Kaplan, and propeller turbines, operate based on both pressure energy and kinetic energy. The water flows through the runner, and the pressure of the water changes as it passes through the blades. At the exit of the runner, the water still possesses considerable kinetic energy and is often at a pressure below atmospheric pressure (especially if the turbine is installed above the tailrace level).

This is where the draft tube becomes essential for reaction turbines. The diverging shape of the draft tube slows down the water, converting the kinetic energy into pressure energy. This process raises the pressure at the runner exit towards the atmospheric pressure in the tailrace, effectively increasing the pressure drop across the turbine runner and allowing the turbine to be placed above the tailrace level without losing the head difference between the runner exit and the tailrace. Without a draft tube, the pressure at the runner exit would be significantly below atmospheric pressure, potentially leading to cavitation, and the kinetic energy leaving the runner would be wasted.

Therefore, the draft tube is specifically designed and used with reaction turbines to ensure efficient operation and energy recovery.

Comparing the options:

Based on the function and principle of operation, the draft tube is specifically associated with reaction turbines.