Cavitation risk in reaction turbines can be mitigated by appropriate design and layout measures. Which of the following collectively help in avoiding cavitation to a great extent?

Introduction:
Cavitation occurs when local pressure drops below vapor pressure, forming vapor bubbles that collapse and damage surfaces. Reaction turbines, especially high-speed runners, are susceptible near low-pressure zones. Design measures must target higher local pressures and better surface resistance.

Given Data / Assumptions:

• Reaction turbine with draft tube.
• Tail race level known; runner elevation selectable.
• Material and surface finish influence erosion resistance.

Concept / Approach:
Submerging the runner (placing it below tail race level) increases static pressure margin against vapor pressure. Stainless steels exhibit better cavitation erosion resistance than plain carbon steels. Polished, smooth runner blades reduce micro-crevice nucleation and suppress boundary layer separation, elevating local static pressure and delaying bubble inception.

Step-by-Step Solution:
1) Increase static head at runner by lowering installation relative to tail water.2) Select cavitation-resistant materials (e.g., stainless steel).3) Achieve smooth surface finish by polishing to reduce nucleation sites and losses.4) Combined, these measures reduce sigma (Thoma cavitation factor) demands.

Verification / Alternative check:
Manufacturer data correlates improved surface finish and material selection with longer cavitation life; submergence calculations show higher absolute pressure at runner exit.

Why Other Options Are Wrong:
Any single measure alone helps but the most comprehensive answer is the combination, reflected in “all of the above”.

Installing above tail water (option e) worsens cavitation by lowering pressure at the runner.

Common Pitfalls:
Focusing only on material while neglecting installation elevation and hydraulic design of the draft tube.

Final Answer:
all of the above