Hydraulic Machines MCQ with answer and solution

Concept

Cavitation threshold and saturation vapor pressure criterion

Formula

p_static ≤ p_vap

Step-by-step solution

1. Cavitation begins when local absolute pressure in a flowing liquid falls to or below its saturation vapor pressure at that temperature. 2. At this threshold, the liquid vaporizes locally and forms cavities or vapor bubbles. 3. When carried into regions of higher pressure, these bubbles collapse violently, pitting metal surfaces and creating vibration. 4. Thus, the inception pressure threshold is the vapor pressure of the liquid. Answer: B

Explanation

Cavitation is the formation, growth, and subsequent violent implosion of vapor-filled cavities in a flowing liquid. It initiates whenever the local absolute static pressure at any point in the system drops to or falls below the saturation vapor pressure of the liquid at the prevailing operating temperature.

Why the other options are wrong

A) Liquid does not boil or cavitate simply below atmospheric pressure; it requires pressure to drop to vapor pressure. C) Stagnation pressure is higher than static pressure. D) Water vapor pressure is positive, so cavitation initiates well above absolute zero.

Shortcut

Cavitation begins when Local Pressure ≤ Vapor Pressure (P_vap).

Common mistake

Assuming cavitation initiates at atmospheric pressure, forgetting that liquids remain in liquid state well below atmospheric pressure.