Cavitation in hydraulic machines Cavitation inception is primarily caused when the local static pressure in the liquid falls below which threshold?

Introduction / Context:
Cavitation produces noise, vibration, erosion, and performance loss in pumps, turbines, and propellers. Recognizing the pressure–velocity interplay that triggers vapour bubble formation is essential for design against cavitation.

Given Data / Assumptions:

• Incompressible liquid (e.g., water) with a known vapour pressure p_v at operating temperature.
• Local static pressure p_s varies along streamlines due to geometry and speed.
• Neglect dissolved gases for the basic argument (though they can exacerbate effects).

Concept / Approach:

By Bernoulli’s principle, higher local velocities often correspond to reduced static pressures. Cavitation starts when p_s dips to p_v (or below), allowing vapour cavity formation. Subsequent collapse in high-pressure regions releases damaging pressure pulses.

Step-by-Step Solution:

Verification / Alternative check:

Use NPSH_available ≥ NPSH_required for pumps to avoid suction cavitation. Empirical cavitation indices (σ) in turbines provide similar checks against p_v limits.

Why Other Options Are Wrong:

(b) High pressure suppresses cavitation. (c) Low velocity tends to increase static pressure, not reduce it. (d) High velocity usually reduces p_s, not increases it; the cause is pressure dropping low, not high.

Common Pitfalls:

Ignoring temperature dependence of p_v; evaluating only average pressure instead of local minima; forgetting inlet losses that reduce suction pressure.

Final Answer:

Low pressure (approaching or below vapour pressure)