Runaway Speed of Hydraulic Turbines — Definition What does the term “runaway speed” of a hydraulic turbine signify?

Introduction:
Runaway speed is a critical safety parameter in hydropower engineering. It defines the highest speed a turbine would attain when the electrical or mechanical load is suddenly removed yet the waterway continues to supply head and flow before the governor throttles the gates.

Given Data / Assumptions:

• Steady head at the turbine inlet.
• Load is removed; inlet guide vanes may still be open momentarily.
• No external braking; negligible mechanical losses for the short transient.

Concept / Approach:
With no load, the resisting torque drops dramatically, so the hydraulic torque accelerates the runner until hydraulic and mechanical losses balance. The resulting equilibrium speed (often 1.6–2.0 times rated, depending on type) is called the runaway speed. Design of shafts, bearings, and generators must account for this contingency.

Step-by-Step Solution:
Assume sudden load rejection → torque imbalance.Runner accelerates due to excess hydraulic torque.Speed rises until losses equal input torque; that speed is the runaway speed under zero load.

Verification / Alternative check:
Manufacturer data specify runaway multipliers for Pelton, Francis, Kaplan; protection systems are sized accordingly.

Why Other Options Are Wrong:
full load/maximum overload: these involve positive shaft power absorption, not free running.no damage: not a definition; survivability is a separate design requirement.

Common Pitfalls:
Confusing synchronous/generator overspeed with the hydraulic runaway condition; concepts are related but not identical.

Final Answer:
at which the turbine will run freely without load