Francis turbine operating significantly below design head Statement: If a Francis turbine is run for long periods at less than 50% of its design/net head, the efficiency drops and the danger of cavitation becomes more serious.

Introduction / Context:
Francis turbines are reaction machines designed to operate around a specific head and discharge. Running far away from this design point—especially at much lower head—changes pressure distribution and velocities inside the runner and draft tube. This not only reduces efficiency but can also aggravate cavitation risk in low-pressure zones.

Given Data / Assumptions:

• Francis (reaction) turbine with spiral casing, stay/guide vanes, runner, and draft tube.
• Operation at < 50% of design/net head for extended periods.
• Speed and guide vane settings adjusted to keep unit on line.

Concept / Approach:
At reduced head, velocities and pressure recovery patterns change. Local static pressures can fall below vapor pressure near the runner exit and within the draft tube, promoting cavitation inception. Off-design incidence on blades increases losses (eddy, separation), so hydraulic efficiency falls. Draft tube swirl and vortex ropes are stronger off-design, further worsening performance and cavitation susceptibility.

Step-by-Step Solution:
Lower head ⇒ lower available specific energy and altered velocity triangles.Blade incidence deviates from optimum ⇒ higher profile and secondary losses.Pressure at runner exit may dip below vapor pressure ⇒ cavitation bubbles form/collapse.Cavitation damages surfaces and introduces unsteady forces ⇒ additional efficiency loss and reliability issues.

Verification / Alternative check:
Unit characteristic curves show peak efficiency near design head; as head is reduced substantially, efficiency drops. Cavitation Thoma coefficient requirements become harder to satisfy at off-design, confirming higher risk.

Why Other Options Are Wrong:
“Disagree” ignores well-known reaction turbine off-design behavior. Pelton (impulse) turbines are different machines. Draft tube presence does not negate low-pressure pockets; even with a draft tube, off-design operation can cavitate. Saying cavitation reduces at low head is contrary to reaction turbine practice.

Common Pitfalls:
Confusing best-efficiency-point shifts with universally high efficiency; assuming lower head always means safer pressures (local pockets can still drop below vapor pressure).

Final Answer:
Agree