Draft tube function and turbine selection criteria

Function of a draft tube

A draft tube is a gradually expanding tube between the runner exit of a reaction turbine and the tailrace, with its outlet submerged. Its functions:

1. Converts the high exit kinetic energy into pressure energy (energy recovery), raising efficiency.
2. Produces a negative head (suction) at the runner exit, so the turbine can be set above tailwater without loss of head.
3. Permits the turbine to be installed above tailwater for easy inspection and maintenance.
4. Discharges water to the tailrace at low velocity.

Selection criteria of hydraulic turbines

1. Net head – the primary guide:

2. Specific speed $N_s = \dfrac{N\sqrt{P}}{H^{5/4}}$ (SI: rpm, kW, m):

The highest permissible $N_s$ is chosen because it gives a smaller and cheaper turbine and generator.

3. Discharge and part-load operation: for widely varying flow (RoR plants), Pelton or Kaplan turbines (flat efficiency curves) or several Francis units are chosen; Francis and propeller turbines lose efficiency below about 50–60% load.

4. Speed: must be a synchronous speed $N = 120f/p$ ($p$ = number of generator poles) for direct coupling; a high speed reduces generator size.

5. Cavitation and setting: Thoma's criterion $\sigma = \dfrac{H_a - H_v - H_s}{H} \ge \sigma_c$ fixes the suction head. High-$N_s$ machines need a deeper setting and more excavation.

6. Sediment and abrasion: in silt-laden Nepalese rivers, turbines whose parts are easy to repair or replace (Pelton buckets, needles) are preferred at medium–high heads; coatings are used for Francis runners.

7. Tailwater variation: reaction turbines with draft tubes use the full head even with varying tailwater; impulse turbines lose the setting height.

8. Number of units: depends on load pattern, flow variation, transport size limits, and reliability (at least two units).

9. Cost and maintenance: combined cost of turbine, generator, powerhouse and excavation; availability of spare parts, local expertise and manufacturer.