Effects of cavitation in hydraulic machines What are the principal adverse effects when cavitation occurs in pumps or turbines?

Introduction / Context:
Cavitation occurs when local static pressure falls to or below the liquid's vapor pressure, leading to the formation of vapor bubbles that subsequently collapse in higher-pressure regions. The implosion of these bubbles generates shock waves and micro-jets that damage surfaces and degrade hydraulic performance.

Given Data / Assumptions:

• Water as the working fluid.
• Operation near cavitation inception conditions (low NPSH, off-design flow, or high local velocities).
• Material surfaces susceptible to pitting under repeated bubble collapse.

Concept / Approach:
Cavitation affects machines mechanically and hydraulically. Mechanically, bubble collapse near solid boundaries produces pitting, erosion, and induces vibration and noise. Hydraulically, vapor cavities displace liquid, altering velocity triangles and limiting effective head transfer, which reduces discharge, head, and overall efficiency. Severe cavitation causes unstable operation and sudden power loss.

Step-by-Step Solution:
As pressure drops below vapor pressure, vapor bubbles form.Bubbles convect to higher pressure zones and collapse violently.Surface damage and unsteady forces arise ⇒ noise and vibration increase.Flow passages partially filled with vapor reduce effective flow area and energy transfer ⇒ discharge, head, and efficiency decrease.

Verification / Alternative check:
Performance curves under cavitating conditions show reduced head-flow capability; inspection reveals pitted blades or volute walls. Vibration spectra exhibit characteristic broadband and discrete components associated with cavity dynamics.

Why Other Options Are Wrong:
Since cavitation simultaneously causes noise/vibration, discharge reduction, and efficiency/power loss, selecting any single effect alone is incomplete; “all of the above” is the correct comprehensive choice. “None of the above” is clearly incorrect.

Common Pitfalls:
Assuming cavitation only causes noise without performance loss; overlooking NPSH requirements and off-design incidence as root causes.

Final Answer:
all of the above