Cavitation in fluid flow occurs under which condition within pumps, valves, or constricted passages?

Introduction / Context:
Cavitation is the formation and subsequent collapse of vapor bubbles in a liquid, a damaging phenomenon in pumps, turbines, propellers, and throttling devices. Recognizing the precise trigger condition helps in design and troubleshooting.

Given Data / Assumptions:

• Liquid with known vapor pressure at the local temperature.
• Flow domain includes regions of high velocity or sharp pressure drops (e.g., impeller eye, valve vena contracta).
• Basic Bernoulli and vapor–liquid equilibrium considerations apply.

Concept / Approach:

As static pressure falls at constant temperature, once it approaches the liquid’s vapor pressure, nucleation of vapor cavities can occur. These cavities convect into higher-pressure regions and collapse violently, producing noise, vibration, efficiency loss, and surface pitting. Hence designs ensure NPSHa > NPSHr and avoid abrupt pressure drops.

Step-by-Step Solution:

Verification / Alternative check:

Bernoulli reasoning: increases in velocity cause static pressure reductions; if the drop is severe, pressure can approach vapor pressure and trigger cavitation. Field indicators include characteristic crackling noise and reduced pump head.

Why Other Options Are Wrong:

Velocity decreases to zero: Not causative; low velocity tends to raise static pressure. Total energy decreases: Vague and not the defining condition. Both: Combines two incorrect/irrelevant statements.

Common Pitfalls:

Using gauge instead of absolute pressure near zero; neglecting temperature dependence of vapor pressure; ignoring suction pipe losses in NPSH calculations.

Final Answer:

When the local flow pressure approaches the liquid’s vapor pressure at the prevailing temperature