A pump handling cold water at 10°C experiences a pressure drop at the impeller inlet. At what point does cavitation begin?
AWhen the water temperature rises above 100°C due to frictional heating
BWhen local pressure drops below the vapor pressure of water at 10°C
CWhen flow velocity exceeds the speed of sound in water
DWhen the pump draws more water than it can discharge, causing backflow
Cavitation is triggered when local pressure falls below the vapor pressure at the prevailing temperature — not when temperature rises to the boiling point at atmospheric pressure. At 10°C, water's vapor pressure is only about 1,230 Pa (much less than atmospheric). If local pressure at the impeller inlet drops below this value due to high velocity (via Bernoulli), the water locally vaporizes and forms bubbles. Cavitation is a pressure-relative-to-vapor-pressure phenomenon, not a thermal boiling phenomenon.
Question 2 Multiple Choice
Where does cavitation damage primarily occur, and what mechanism causes it?
AAt the point of lowest pressure, where bubbles form and physically erode the surface
BUniformly across the wetted surface, as vapor bubbles abrade the material
CIn high-pressure zones downstream, where collapsing bubbles generate intense pressure spikes and microjets
DAt the pump inlet, where turbulent flow creates direct mechanical impact
Cavitation damage occurs downstream, where cavitation bubbles travel into higher-pressure regions and violently implode. The collapse is asymmetric: liquid rushes inward faster than the speed of sound, generating focused microjets and pressure pulses reaching thousands of atmospheres. These repeated impacts pit and erode the surface. The formation of bubbles causes little direct damage — the danger is the collapse. This is why cavitation damage appears on impeller blades and turbine runners in zones of flow reattachment, not at the suction inlet.
Question 3 True / False
Cavitation can occur in cold water at temperatures well below 100°C if local pressure drops low enough.
TTrue
FFalse
Answer: True
Boiling at atmospheric pressure requires 100°C, but cavitation is vaporization at reduced pressure — the phase transition occurs whenever local pressure falls below the vapor pressure at the current temperature. At 10°C, water's vapor pressure is about 1,230 Pa; at 20°C, about 2,300 Pa. In a fast-flowing pump or turbine, the Bernoulli effect can reduce local pressure to these levels even in cold water. The thermodynamics are the same: vapor pressure depends on temperature, and vaporization occurs whenever ambient pressure drops below it.
Question 4 True / False
Cavitation damage is caused primarily by the formation of vapor bubbles, which create voids that weaken the surface material.
TTrue
FFalse
Answer: False
Formation of bubbles is not the damaging event — collapse is. When a cavitation bubble moves from a low-pressure zone into a higher-pressure zone, it implodes violently and asymmetrically. The surrounding liquid collapses inward, producing microjets directed at the nearby surface with pressures reaching thousands of atmospheres. These repeated micro-impacts fatigue and pit the surface over time. The appearance resembles sandblasting from the inside. Understanding that damage comes from collapse (not formation) explains why damage occurs on the downstream, high-pressure faces of impeller blades.
Question 5 Short Answer
Explain why cavitation is described as a 'pressure-relative-to-vapor-pressure' problem rather than simply a boiling or overheating problem, and what this means for prevention.
Think about your answer, then reveal below.
Model answer: Cavitation and boiling are the same phase transition (liquid to vapor) but triggered by different mechanisms. Boiling is caused by raising temperature until vapor pressure exceeds ambient pressure; cavitation is caused by lowering local pressure (via high-velocity flow) below vapor pressure at the existing temperature. Prevention therefore targets the pressure margin: increase static pressure at the problem location, lower fluid temperature (which reduces vapor pressure), reduce velocity, or raise inlet head. Simply cooling the fluid is not always practical, but raising inlet pressure or reducing flow speed directly addresses the root cause.
The cavitation number σ = (P − P_v)/(½ρV²) formalizes the 'pressure margin' concept: the numerator is how far local pressure exceeds vapor pressure, and the denominator is the dynamic pressure. A high σ means cavitation is unlikely; a low σ signals risk. Prevention strategies all increase σ: raising P (inlet pressure), lowering P_v (cooler fluid), or reducing V (lower velocity). This framing also explains why NPSH (Net Positive Suction Head) specifications exist — they guarantee that inlet pressure stays above vapor pressure with a safety margin.