Cavitation Number Calculator

What is Cavitation?

Cavitation occurs when local fluid pressure falls below the vapour pressure of the liquid. Vapour bubbles form, and when they collapse (implode) near a surface, they release enormous localised energy, causing pitting, erosion, noise, and vibration. It destroys pump impellers, propellers, and control valves.

The Cavitation Number

The cavitation number (σ) quantifies the margin between operating pressure and vapour pressure:

σ = (P − Pv) / (½ρv²)

Where:

• P = local static pressure (Pa)
• Pv = vapour pressure of liquid at operating temperature (Pa)
• ρ = fluid density (kg/m³)
• v = local fluid velocity (m/s)

Higher σ = safer from cavitation. Cavitation begins when σ drops to the critical value σ_c (device-specific).

Vapour Pressure of Water vs Temperature

| Temp (°C) | Vapour Pressure (kPa) | |-----------|----------------------| | 20 | 2.34 | | 40 | 7.38 | | 60 | 19.9 | | 80 | 47.4 | | 100 | 101.3 |

Practical Example

Pump inlet: P = 120 kPa, water at 20°C (Pv = 2.34 kPa), v = 5 m/s. σ = (120,000 − 2,340) / (0.5 × 1000 × 25) = 117,660 / 12,500 = 9.41 — well above critical for most pumps.

Prevention

• Increase inlet pressure (NPSH margin).
• Reduce fluid temperature.
• Reduce velocity at the low-pressure point.
• Use materials resistant to cavitation erosion (stainless steel, bronze).