Cavitation in Hydraulic Machines Cavitation in turbines and pumps primarily results from which condition in the flowing liquid?

Introduction:
Cavitation is the formation and collapse of vapor bubbles in a liquid when local static pressure falls below vapor pressure. It causes pitting, noise, vibration, drop in efficiency, and potential structural damage in hydraulic machinery.

Given Data / Assumptions:

• Working fluid is water at typical operating temperature.
• Local static pressure can vary due to velocity changes and elevation head.
• Vapor pressure is a known function of temperature.

Concept / Approach:
Bernoulli’s principle indicates regions of high velocity can have low static pressure. If this pressure dips below vapor pressure, vapor cavities form. Downstream where pressure recovers, these bubbles collapse violently on surfaces, producing cavitation damage and performance losses.

Step-by-Step Solution:
Identify low-pressure zones (e.g., pump suction, runner exit, draft tube throat).Compare local pressure to vapor pressure at operating temperature.If p_local < p_vapor, cavitation inception occurs.

Verification / Alternative check:
Net Positive Suction Head (NPSH) calculations predict cavitation margins in pumps; Thoma coefficient sigma does similarly for turbines.

Why Other Options Are Wrong:
low velocity: generally raises static pressure; not the cause.high velocity: contributes indirectly only when it causes low pressure; the root cause is pressure below vapor pressure.high pressure: opposite of what triggers cavitation.

Common Pitfalls:
Assuming velocity alone causes cavitation; it is the associated pressure drop that matters.

Final Answer:
low pressure