Questions: Cavitation Number and Cavitation Prediction

Question 1

A water pump operating at sea level (atmospheric pressure ~101 kPa) is relocated to a mountain site at 2000 m altitude where atmospheric pressure is ~80 kPa. All other installation conditions remain the same. What happens to cavitation risk?

• A. Risk decreases — thinner air at altitude reduces the density of the fluid, lowering dynamic pressure
• B. Risk increases — lower atmospheric pressure reduces NPSH_available, shrinking the margin above vapor pressure
• C. Risk is unchanged — NPSH_required is a pump property set by the manufacturer, independent of installation
• D. Risk decreases — the cooler temperatures at altitude reduce vapor pressure, providing more margin

Answer: (B)

NPSH_available = (P_inlet/ρg + V²/2g) − P_vapor/ρg. If atmospheric pressure drops by 21 kPa (≈ 2.1 m of water head), the absolute pressure at the pump inlet falls by the same amount, directly reducing NPSH_available. The vapor pressure of water at the same temperature is essentially unchanged. The safety margin against cavitation shrinks by 2.1 m of head. Option C is wrong because NPSH_required is indeed a pump property, but NPSH_available depends on the installation — and it's the comparison between the two that determines cavitation risk.

Question 2

A pump handling water at 80°C (vapor pressure ≈ 47 kPa) instead of 20°C (vapor pressure ≈ 2.3 kPa) operates at the same inlet pressure. What is the primary effect on cavitation risk?

• A. Risk decreases — hot water is less viscous, reducing frictional losses at the pump inlet
• B. Risk is essentially unchanged — vapor pressure is a material property, not an operational variable
• C. Risk increases significantly — the margin between inlet pressure and vapor pressure has shrunk by roughly 45 kPa
• D. Risk increases slightly — higher temperature causes minor changes in fluid density

Answer: (C)

NPSH_available includes the term −P_vapor/ρg. At 20°C, P_vapor ≈ 2.3 kPa, contributing roughly 0.23 m to the head subtracted. At 80°C, P_vapor ≈ 47 kPa, contributing about 4.8 m. NPSH_available drops by roughly 4.6 m of head — a dramatic reduction in margin even though nothing changed about the piping system. This is why pumps handling hot fluids, boiler feed water, or liquids near their boiling points require careful NPSH analysis and often need to be positioned with significant positive suction head.

Question 3

A higher cavitation number σ indicates that the system is closer to the cavitation threshold and at greater risk of bubble formation.

• T. True
• F. False

Answer: False

The cavitation number σ = (P − P_vapor) / (½ρV²) measures the pressure margin above vapor pressure, normalized by dynamic pressure. A higher σ means there is more pressure 'headroom' before the local pressure drops to vapor pressure — a larger safety margin, lower cavitation risk. Cavitation inception occurs when σ falls below the critical value σᵢ. The misconception of reversing this direction is common; remember that σ is a ratio of available margin to dynamic pressure, so more is safer.

Question 4

For a centrifugal pump at fixed inlet conditions, increasing the flow rate typically increases cavitation risk.

• T. True
• F. False

Answer: True

Two effects combine to worsen cavitation as flow rate increases. First, NPSH_required (a property of the pump) increases with flow rate — the pump demands more inlet head to operate without cavitation at higher throughput. Second, higher flow velocities at the inlet lower local static pressure via Bernoulli's principle, reducing the actual margin above vapor pressure. Both effects simultaneously narrow the gap between NPSH_available and NPSH_required, pushing the system toward the cavitation threshold.

Question 5

Why does operating temperature matter so much when assessing cavitation risk, and what physical property makes pumps handling hot fluids especially vulnerable?

Model answer: Vapor pressure increases rapidly — and nonlinearly — with temperature. At 20°C, water's vapor pressure is about 2.3 kPa; at 100°C it equals atmospheric pressure (101 kPa). NPSH_available subtracts the vapor pressure head from the absolute inlet pressure, so higher vapor pressure directly reduces NPSH_available. For hot fluids, this subtraction becomes large, leaving little or no margin above cavitation threshold even when inlet pressures seem adequate. Pumps near the boiling point of the working fluid are most vulnerable because virtually any local pressure drop will trigger vaporization.

The physical mechanism is that vapor pressure represents the pressure at which the liquid spontaneously vaporizes at that temperature. Any local pressure minimum in the flow (over a blade, at a throat, around an impeller tip) that dips below vapor pressure causes immediate bubble nucleation. As temperature rises, vapor pressure rises, so the 'danger zone' of pressures expands — the entire pressure range between vapor pressure and the operating pressure becomes safe territory that shrinks with temperature.