Hydraulic Turbines — Draft Tube Usage Is a draft tube used with impulse turbines such as the Pelton wheel, or is it associated with reaction turbines only?

Introduction:
A draft tube is a gradually expanding conduit fitted at the outlet of reaction turbines. Its purpose is to convert part of the kinetic energy at runner exit into useful pressure energy and to allow installation of the turbine above the tailrace without losing head. The question tests whether this device is relevant to impulse turbines like Pelton wheels.

Given Data / Assumptions:

• Impulse turbine (e.g., Pelton) exhausts water to atmosphere after striking buckets.
• Reaction turbines (e.g., Francis, Kaplan) discharge with significant velocity head at runner exit.
• Steady incompressible flow; standard textbook definitions of turbine components.

Concept / Approach:
Impulse turbines operate at essentially atmospheric pressure throughout the runner and extract energy solely from the change in jet momentum. There is no useful pressure recovery possible downstream; water simply falls into the tailrace. Reaction turbines, however, discharge at sub-atmospheric pressure with high exit velocity; a draft tube recovers kinetic energy by diffusion and raises the static pressure closer to tailrace level, improving overall efficiency.

Step-by-Step Solution:
Identify turbine type: impulse vs reaction.For impulse: runner exit is atmospheric; no pressure rise is feasible via a diffuser.For reaction: runner exit velocity is high; draft tube converts velocity head to pressure head.Conclusion: draft tube is not used with impulse turbines.

Verification / Alternative check:
Plant layouts show Pelton tailraces without draft tubes, while Francis/Kaplan runners connect to elbow or conical draft tubes.

Why Other Options Are Wrong:
Yes: contradicts operating principle of impulse machines.Only in high-head plants: head magnitude does not change the impulse principle.Only for speed control: draft tubes are not speed-control devices.

Common Pitfalls:
Assuming all water turbines require draft tubes; only reaction types do.

Final Answer:
No